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Singapore 6th Grade Math Lesson 8
Whole Numbers

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Math Lesson 8: Pet Monster

Cindy and Mandy wanted to buy the ugly blob monster in the pet shop. They had a total of $543 with them on Monday but it was not enough to buy the monster. So Cindy's mother gave her $57 on Tuesday. Mandy had 3 times as much money as Cindy after that, and the delighted girls had enough money to buy the pet. They skipped over to the shop and bought the pet on Wednesday. How much money did each girl have on Monday?

Let's Solve It!

Please draw the following diagram on a piece of paper. By drawing along with me, you will be better able to understand how I solve this problem.

Let's draw the model for Tuesday rather than Monday because the girls had the most amount of money on Tuesday.

This is how the model will look like on Tuesday. After Cindy's mother gave Cindy $57, Mandy had 3 times as much money as Cindy. That is why Mandy's bar is 3 times as big as Cindy's. Also notice that I have put in the $57 in Cindy's bar. And finally, I have written the total which is $543 + $57.

The total amount of money the girls had on Tuesday was:
$543 + $57 = $600

If you look at the model, you will see that there are 4 units altogether - Cindy has 1 and Mandy has 3.

How much is 1 unit worth?
$600 ÷ 4 = $150

I shall write that down in the model. See the text in red.

We can now figure out how much money each girl had on Tuesday:

Cindy on Tuesday: $150 (her whole bar equals 1 unit)

Mandy on Tuesday: $150 x 3 units = $450

Now what about Monday?

Mandy's figure remains the same so it is still $450.

Cindy's figure changes because she had $57 less on Monday. Remember that Cindy's mother gave her $57 on Tuesday, which is the next day. So let's deduct $57 from Cindy.

Cindy on Monday: $150 - $57 = $93

Look at the following diagram. I have written $93 in red there.

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6th Grade: Lesson 8 Answer

So the final answer:
On Monday, Cindy had $93 and Mandy had $450.

It may look a bit difficult but if you read the lesson a few times, I think you will be able to understand it. Then it's your turn to solve 3 Interactive Math Problems.

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