City-Data Forum Need help solving a math problem (ADN, middle school, mathematics, master)
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09-22-2009, 12:28 PM
 831 posts, read 1,349,328 times Reputation: 2361

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I am wanting to get my daughter a workbook with word problems. So I am looking at Singapore math 3rd grade. For the life of me I can't solve this sample problem.

There are 3 fish tanks A B and C. B weighs 6 times as much as A and twice as much as C. If C is 36 lb heavier than A find the total weight of A B and C.

That's it. I think I need another number or something to solve this. And if can't do it how in the world can a 3rd grader do it.

Thanks for your help.

09-22-2009, 12:36 PM
 Location: Whoville.... 25,393 posts, read 30,724,367 times Reputation: 14583
Three equations and three unknowns.

B = 6A
B = 2C
substituting : C = 3A

You also know that A+36 = C

substituting A + 36 = 3A (subtracting A from both sides : 36 = 2A, Divide both sides by 2 and you get A = 18)

Solving for A : A = 18
Substituting A intoe equation one adn then B into equation two at the top if this post you get:

B = 108 and C = 54

If I did my math right, the total mass is 180 lbs

Seems a little complex for a 3rd grade book but my daughters went through Singapore math and did fine. My older daughter was allowed to skip algebra alltogether when she transferred in 9th grade and my younger daughter was placed in algebra in 7th grade.

I've read that Singapore math is one year ahead of our traditional math programs by 2nd grade and 2 years ahead by 5th.

My daughters transferred into Singapore math in 2nd and 5th grades respectively. Dd#1 was considered a year behind when she started Singapore math but was a year a head going into her second year of the program. I have no doubt that dd#2 could have solved this problem in 3rd grade. I don't think dd#1 could have but she was in a traditional program until 5th grade so she wouldn't have been exposed to algebra that early.

Last edited by Ivorytickler; 09-22-2009 at 12:52 PM..

09-22-2009, 12:52 PM
 Location: Columbus, Indiana 957 posts, read 1,940,188 times Reputation: 1347
I believe A=18
3A = A + 36
3A - A = 36
2A = 36
A = 18
total weight is 180

09-22-2009, 12:56 PM
 Location: Whoville.... 25,393 posts, read 30,724,367 times Reputation: 14583
Quote:
 Originally Posted by Cabe09 I believe A=18 3A = A + 36 3A - A = 36 2A = 36 A = 18 total weight is 180
That's what I got the second time. I punched something in wrong the first time.

09-22-2009, 12:56 PM
 831 posts, read 1,349,328 times Reputation: 2361
Thank You so much. This was driving me crazy.

And Drummerboy I think you need to get a life.

Last edited by SuzyQ123; 09-22-2009 at 01:07 PM..

09-22-2009, 01:44 PM
 2,039 posts, read 4,903,511 times Reputation: 1204
Amazying how many people are good at solving math, I wish I had such talent. What is Singapore math?

Last edited by LovingSAT; 09-22-2009 at 02:02 PM.. Reason: Great post, lots of learning experience! Thanks.

09-22-2009, 02:58 PM
 817 posts, read 1,430,677 times Reputation: 1476
I think this is a good break from the typical posts. If we are on Education it means we like learning so I think there should be more problems like this.

I am a teacher and ordered Singapore Math just for myself to see if I could use it in the classroom. I have 5th and 6th Grade books, but they have very similar problems.

The algebraic solution is correct, but that is not how they want the students to solve the problems. They don't expect a 3rd Grader to solve it using Algebra. They expect them to use pictures , namely rectangles of different sizes. It should have some examples that show you how to solve these types of problems.

Please don't start teaching a third grader using variables, they need to understand it through pictures first which will build a foundation when they do move to Algebra.

This is how I would start the problem.

You should have a long rectangle that represents B.

Underneath that the A should be a rectangle of the same size, but broken into 6 pieces.

Underneath that the C should be a rectangle of the same size, but broken into 2 pieces.

They should be able to see that one of the C rectangles equals 3 of the A rectangles.

The last one should have a little A rectangle the same size as the small pieces from the original long rectangle. Attached to it can be a block that represents 36 making both of them the same size as a C rectangle.

The student can see that the A rectangle plus the 36 block is also equal to 3 A rectangles since they are both the same length as one C rectangle, therefore two A rectangles are equal to 36 and one A rectangle is equal to 18.

It's kind of hard to understand unless I showed you the picture. You have to try to draw it out.

This is how they expect the students to solve the problems before the reach Algebra, thereby laying a foundation of understanding. A lot of time parents just want to show their children how to do something and don't understand that a foundation has to be laid to be built upon later. Variables should definitely not be used to solve this problem with a 3rd Grader.

09-22-2009, 03:01 PM
 9 posts, read 25,517 times Reputation: 10
Well, that is not how you are supposed to solve it using the methods taught in the Primary Mathematics (Singapore math). They don't teach algebra in third grade, but they do teach solving these kinds of problems with a visual bar model. So if you do come to a forum about city data with a math question, you may not get the best answer. So Drummerboy does have a point.

09-22-2009, 03:45 PM
 817 posts, read 1,430,677 times Reputation: 1476
My explanation was the visual bar model.

09-22-2009, 06:23 PM
 Location: Whoville.... 25,393 posts, read 30,724,367 times Reputation: 14583
Quote:
 Originally Posted by LovingSAT Amazying how many people are good at solving math, I wish I had such talent. What is Singapore math?
Singapore math is a math program used in Asia that has proven results. It's not popular here because it doesn't spiral. They take the approach that if you teach something to mastery the first time, you don't have to keep rehandling it year after year. Teachers like it because it requies less time to teach math.

The time savings comes from not rehandling topics as much. Every time you bring a topic back, you have to review it, bring your students back up to speed and then go on. In the United States, we repeat math topics for 3-8 years. Singapore math only repeats them for 1-3 years. It takes them longer to get to all of the topics but by the time they do, kids have mastered them.

We cover, something like, 14 math topics a year. They cover something like 7 or 8. They stay on each topic longer and go deeper. They give kids time to learn to mastery.

My kids used to have a ton of math homework and requests from the school for drillwork before they transferred into the charter teaching Singapore math. While my kids did have to work to catch up, once they did, I never saw math homework again.

I can't argue with the results. I have two kids who test a minimum of two years ahead in math and think math is easy and my oldest daughter was only in Sinapore math fore two years (of course there was some make up work as they started her a year below grade level coming in so she's had the equivalent of three years just taught in two (they switched to Scott Foresman in middle school))

Oh, and Sinapore math is one of the cheapest programs out there. Unfortunately, we're so convinced we can reinvent the wheel and do it better, we refuse to learn from those who already teach math well. Of course the student is required to work to keep up which is taboo in America where we bend over backwards to make sure kids don't actually have to put effort into their own educations. :shrug
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