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2nd grade math problem

Old 04-16-18, 07:47 PM
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2nd grade math problem

Without using division how would you come up with the answer?

The perimeter of a rectangle is 48cm. One side is 14cm. How long are the other sides?

In my mind you subtract 48 by 28 and then take half of 20. However if you don't know what half is or how to divide be 2 how would you find the length of the other 2 sides?
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Old 04-16-18, 07:56 PM
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Re: 2nd grade math problem

BRB

Let me ask my 5th grader kid.
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Old 04-16-18, 08:05 PM
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Re: 2nd grade math problem

2X + 2Y = 48

X = 14

2 x 14 +2Y = 48

28 +2Y = 48

2Y = 48-28

2Y = 20

Y = 20/2

Y = 10.

I had no idea they were teaching algebra to second graders. I wish they had done that back in ye olden tymes when I were a wee tot.
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Old 04-16-18, 08:14 PM
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Re: 2nd grade math problem

I was told there would be no math.
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Old 04-16-18, 08:20 PM
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Re: 2nd grade math problem

Originally Posted by Jason View Post
2X + 2Y = 48

X = 14

2 x 14 +2Y = 48

28 +2Y = 48

2Y = 48-28

2Y = 20

Y = 20/2

Y = 10.

I had no idea they were teaching algebra to second graders. I wish they had done that back in ye olden tymes when I were a wee tot.
There is still a division in there. If they havenít learned fractions or division Iím not sure how theyíd get the answer.
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Old 04-16-18, 08:26 PM
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Re: 2nd grade math problem

Originally Posted by Timber View Post
Without using division how would you come up with the answer?

The perimeter of a rectangle is 48cm. One side is 14cm. How long are the other sides?

In my mind you subtract 48 by 28 and then take half of 20. However if you don't know what half is or how to divide be 2 how would you find the length of the other 2 sides?
Why would a person NOT use division.
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Old 04-16-18, 08:33 PM
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Re: 2nd grade math problem

Originally Posted by DVD Polizei View Post
Why would a person NOT use division.
Because they havenít been taught division.
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Old 04-16-18, 08:37 PM
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Re: 2nd grade math problem

Yeah, it's very simple division but you still need it to divide the remaining 20 cm by two. If there's a way to figure this out without using any division at all I can't think of it.
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Old 04-16-18, 09:09 PM
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Re: 2nd grade math problem

4th grade teacher here. Obviously, I can't speak for all teachers, especially 2nd grade teachers, but it's my understanding the intent of this type of question for a 2nd grader is to figure out the lengths of the other two sides by using division but not in the rote sense that an adult might go to like the algebraic equations Jason used. On the last step, the student might ask "What's half of 20?". While this is technically division, it's the concept of fractions' connection to division that's probably the intent of the problem. I'm pretty sure many if not most 2nd graders could figure out half of 20 - at least in my teaching experience.
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Old 04-16-18, 09:21 PM
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Re: 2nd grade math problem

Wow I need to go back to middle school
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Old 04-16-18, 09:40 PM
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Re: 2nd grade math problem

______
14|_____|14

14+14=28

48-28=20

So you have 20 and both sides need to be equal.

1+19=20
2+18=20
3+17=20

. . .

10+10=20 ---here's the answer, because both sides have to be the same since rectangles have opposite sides of the same length.
9 +11
8+12 etc

___10___
14|________|14
10

That's how I'd do it.
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Old 04-16-18, 09:46 PM
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Re: 2nd grade math problem

I thought the thread was "2nd grade meth problem".

I am so disappointed.
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Old 04-16-18, 09:50 PM
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Re: 2nd grade math problem

wow.. The Flynn effect must be alive and thriving, or all the kids (yes, 7 year-old KIDS) in that class must have IQs in the "very superior" range.

That sounds like a 5th or 6th grade problem to me. I don't think I even learned about perimeter until at least 3rd or possibly even 4th grade, and this was in the "gifted" class in math. Let alone how to compute what amounts to an algebraic problem using unknowns and "division" like this one!

So, they're really pushing things these days, it seems.

Or, teaching by some alien form (core?) which I guess tries to expose kids to as much as possible as early as possible in putting the whole picture of math together.
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Old 04-16-18, 11:25 PM
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Re: 2nd grade math problem

Sum of the other two sides = 20.
We know two 14cm sides give us 28, so the unknown side must be less than or equal to 13.
So now just go through 13, 12, 11,...till you get a number which when doubled gives 20.
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Old 04-17-18, 12:22 AM
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Re: 2nd grade math problem

Simplest way without dividing is to straight up make the rectangle yourself:

You could use graph paper (with two sides known, brief trial and error for the other two) and count out the perimeter.

If you want to explain it with blocks for a more hands on approach: You can lay down 14 and 14 for the top and bottom of the rectangle, and have 28. Then you put one block for each side and count out in pairs (29,30; 31,32) until you get to 48 and both sides “connect”, forming the rectangle. Then you can count how many blocks you used to go down each side. You can do a final check and count all the way around to 48 again.

Last edited by bluetoast; 04-17-18 at 12:30 AM.
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Old 04-17-18, 06:48 AM
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Re: 2nd grade math problem

Originally Posted by Timber View Post
Without using division how would you come up with the answer?

The perimeter of a rectangle is 48cm. One side is 14cm. How long are the other sides?

In my mind you subtract 48 by 28 and then take half of 20. However if you don't know what half is or how to divide be 2 how would you find the length of the other 2 sides?
No expert, but they do all this "number line" stuff. Mark off 20 cm on a piece of paper, fold so the ends of the line match, measure from either end to the fold. You have just divided by two without understanding the division process.

With more folds, works great for division by 4, 8, and so on.

Or by trial and error. You need two equal numbers that add to 20. Guess (wrong), like 8, subtract from 20 to get 12, not quite equal. But you know the number is bigger than 8, smaller than 12, Repeat. You quickly get to 10 and 10. (For more complicated problems, this becomes Newton's method, but that requires calculus and derivatives, and they can't even divide yet)
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Old 04-17-18, 07:20 AM
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Re: 2nd grade math problem

Without any division?

Though I would consider knowing that half of twenty is to ten to barely qualify as division, you could probably physically construct a rectangle out of blocks or tiles, and then count the units out.

ETA: bluetoast beat me to the punch.
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Old 04-17-18, 07:23 AM
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Re: 2nd grade math problem

Originally Posted by tasha99 View Post
______
14|_____|14

14+14=28

48-28=20

So you have 20 and both sides need to be equal.

1+19=20
2+18=20
3+17=20

. . .

10+10=20 ---here's the answer, because both sides have to be the same since rectangles have opposite sides of the same length.
9 +11
8+12 etc

___10___
14|________|14
10

That's how I'd do it.
Somebody knows Common Core math.
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Old 04-17-18, 07:23 AM
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Re: 2nd grade math problem

Originally Posted by Josh-da-man View Post
Without any division?

Though I would consider knowing that half of twenty is to ten to barely qualify as division,
That's where I'm leaning. They're expected to know that without knowing how they get there. Although I asked her what half of 20 was and she said she didn't know what half meant. She then answered 5 when I asked what half of 10 was. Maybe it was a bonus question.

edit: But...you can "show your work" without dividing by 2.

Last edited by Timber; 04-17-18 at 07:29 AM.
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Old 04-17-18, 07:42 AM
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Re: 2nd grade math problem

WTF they trying to teach 2nd graders this shit for? For a kid 7 years old, seems like they're trying too hard for college prep.

In my mind, if this is a 48cm rectangle and one side is 14cm, the opposite side has to be 14cm as well. 14+14=28. 48-28=20. The other 2 sides share that 20, so 10 each. Without simple division, and without some complicated fucking algebra (for a 7 year old), I'd have a harder time solving the problem.
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Old 04-17-18, 07:52 AM
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Re: 2nd grade math problem

I was thinking blocks like mentioned, or just simply tally marks.sketch out the rectangle then put one tally on one side, then one on the other side. Alternate until you get to twenty
Originally Posted by Josh-da-man View Post
Though I would consider knowing that half of twenty is to ten to barely qualify as division:
Gotta show your work.
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Old 04-17-18, 08:42 AM
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Re: 2nd grade math problem

My son is in the 2nd grade, and with some prodding, I got him to work the problem.

Two of the sides are 14.
He didn't know 14+14, but he does know 15+15 = 30. So 14+14 = 30-2 = 28.
Then 48-28 = 20
And 10+10 = 20, so the other two sides are each 10.
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Old 04-17-18, 08:48 AM
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Re: 2nd grade math problem

Straight from the brain of a 7 year old. I think that's the answer, or method, the question was going for.
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Old 04-17-18, 09:01 AM
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Re: 2nd grade math problem

It's so simple
So very simple
That only a child can do it!
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Old 04-17-18, 10:07 AM
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Re: 2nd grade math problem

Easy.
Instead of dividing by 2, multiply by 0.5

"2Y = 20"
"(2Y x 0.5) = (20 x 0.5)"
"Y = 10"

You might think this is cheating, but no part of the rules said you can't multiply by half.
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