Q&A - PSLE Math

[youngmum]

lizawa - i stil don’t understand still leh. So sorry. Got easier explanation?


How to do this?

May earned twice as much money as her brother. After May spent $32 and her brother spent $50, the ratio of money May had left to the amount of money her brother had left was 7:3. Find the amount of money May earned.

Thanks.

[corneyAmber]

youngmum, solution for new question as follows:


May ____________________________ ____________________________

May {---------32-------}{----------------------7 units-----------------------------------}

Bro ____________________________

Bro {------50-------------}{----3units----}

So the equation is:

32 + 7 units = 2(50 + 3 units)
32 + 7 units = 100 + 6 units
1 unit = 100-32 = 68

Hence May earned 7(68 )+32 = $508

[corneyAmber]

The other question is changing % into fractions that lizawa has cleverly done as it is easier to think in 1 unit than as 20% or 25%.

20% = 1 in 5 parts
25% = 1 in 4 parts

it is known total people = 1150
total non-photographers = 250
20% men and 25% women = 250 so in fraction:
1 unit of men + 1 unit of women = 250

since 1 unit of men and 1 unit of women are different things, it is hard to make comparison, however, we know that if we take away 4 units from men and women, the remainder is 1 unit of men.

hence 2504 = 1000
1150-1000 = 150
Only then you will know 1 unit of men(=20% of men) = 150

so total men(100% of men) = 1505 = 750

1 unit of women(=25% of women) = 250-150 = 100

so total women (100% of women) = 100*4 = 400

hence difference between men and women = 750-400=350

[ChiefKiasu]

Sarah:

...Q1. Bobby, Edward and kelvin made a bottle of paper stars. The number of paper stars Bobby made was 21 more than 1/4 the total number of paper stars in the bottle. The number of paper stars Edward made was 30 more than 1/4 of the remaining number of paper stars in the bottle. Kelvin made the remaining 114 paper stars. How many paper stars did they make in all?

Start backwards from Edward. Since Edward made 30 more than a quarter of the stars and Kelvin made the rest, it means that Kelvin's stars plus 30 makes up 3/4 of the stars,
ie. 114 + 30 = 144 is 3/4 of the stars
So the sum of Edward's and Kelvin's stars is 144 x 4/3 = 192

We apply the same principle with Bobby's stars now.
3/4 of the total stars = 192 + 21 = 213
So the total number of stars made by all 3 children is 213 x 4/3 = 284

Sarah:

...Q2. A, B and C are three numbers. The average of A, B and C is 5 greater than the average of A and B. If C is 38, find the total of A, B and C...

(A+B+C)/3 = (A+B)/2 + 5
A+B+C = 3(A+B)/2 + 15
A+B+38 = 3(A+B)/2 + 15
A+B+23 = 3(A+B)/2
2(A+B)+46 = 3(A+B)
=> A+B = 46
=> A+B+C = 46 + 38 = 84

[youngmum]

Thanks ks2me. I’ll slowly digest and see if i can catch the ball. kiss

[lizawa]

youngmum:

Thanks ks2me. I'll slowly digest and see if i can catch the ball. *kiss*

Yes, ks2me's explanation is correct. It's just converting % to fractions. You can show your child the solution and see if he understands. The kids are so trained to using models to solve such questions, sometimes, you just need to help them to start with the correct model, and they can \"see\" the solution.

[Sarah]

thanks Chief for the working.


Need help for the following $ questions.

Question 1
the ration of $2 notes to 50-cent coins in Mabel’s piggy bank was 13:8. She exchanged 24 pieces of $2 notes for some 50-cent coins, after which, the ratio of $2 notes to 50-cent coins became 1:8. What was the total value of $2 notes and 50-cent coins in the piggy bank at first?

Question 2
the ration of $5 notes to 20-cent coins in Mr Tan/ counter was 19:5. Mr Tan exchanged 15 of his $5 notes for some 20-cent coins. The ratio of the $5 notes to 20-cent coins then became 1:5. What was the total value of $5 notes and 20-cent coins at first?

Question 3
a box contained 50-cent coins and 20-cent coins in the ratio of 2:3. Peter took out four 50-cent coins, exchanged them for 20-cents and put the money back into the box. The ratio then became 2:7. Find the sum of money in the box at first.

thanks for helping, exam is coming, u know.

[Priskhipo]

HI Sarah,


The 3 questions share similar strategy.

http://www.postimage.org/image.php?v=gx1Rl8GA

http://www.postimage.org/image.php?v=Pq25SceS

http://www.postimage.org/image.php?v=Pq25SrcJ

[youngmum]

For the above question 1 - for step 1 why 12 + 24? I don't understand the first step, the rest ok. Pls explain to me. Thanks.


i solved it the ratio way though.

13: 8 = 39 : 24
1:8 = 15:120

So, 39 x 2 = 78
24/2 = 12

total = 78 + 12 = 90

[ChiefKiasu]

Priskhipo:

HI Sarah,


The 3 questions share similar strategy.
...

Priskhipo, thanks for the detailed \"workout\"!