## Q&A - P5 Math

### Re: Q&A - P5 Math

Thank you for your prompt reply. If it is not too troublesome, the following is my questions to clear my understanding..

The ratio of the boys 5A : 5B, the unit size (boys), might not be the same as the unit size (girls) of the ratio of the girls 5A : 5 B. Unless, for example, I said the ratio of blue pen in dozen to red pen is 1 dozen of blue pen to 2 red pens, by specifically mention the unit of measurement in the ratio statement. Correct ?

So, if we assume they are the same, can we then said the total no. of the students in 2 classes make up of 14 parts. If so, the total number of students for 5A and 5B are 42+42=84. Then each part is 84/14=6. So the total number of boys will be 6 x 7 = 42 ?

Kindly advise as I know I have a wrong concept in thinking in this manner. But do not understand which part I get "wrong"

The ratio of the boys 5A : 5B, the unit size (boys), might not be the same as the unit size (girls) of the ratio of the girls 5A : 5 B. Unless, for example, I said the ratio of blue pen in dozen to red pen is 1 dozen of blue pen to 2 red pens, by specifically mention the unit of measurement in the ratio statement. Correct ?

So, if we assume they are the same, can we then said the total no. of the students in 2 classes make up of 14 parts. If so, the total number of students for 5A and 5B are 42+42=84. Then each part is 84/14=6. So the total number of boys will be 6 x 7 = 42 ?

Kindly advise as I know I have a wrong concept in thinking in this manner. But do not understand which part I get "wrong"

Last edited by vaun on Thu Jul 23, 2015 1:34 pm, edited 1 time in total.

- alfretztay
- KiasuGrandMaster
**Posts:**1199**Joined:**Sun Sep 12,**Total Likes:**5

### Re: Q&A - P5 Math

I believe that the sentence 'There were 56 boys altogether in both classes.' ought to be deleted in order to arrive at the standard answer. Alternatively, '56' could be changed to 42.numeric wrote:vaun wrote:P5 Math Mid year Exam 2010 - Question 17

There are 42 pupils in each of the classes. primary 5A and primary 5B. The ratio of the no. of boys in 5A to the no. of boys in 5B was 3:4. The ratio of the no. of girls in 5A to the no. of girls in 5B was 4:3. There were 56 boys altogether in both classes. What was the ratio of boys in 5A to the no. of girls in in 5B.

My steps :

......5A (42) : 5B(42)

Boy Ratio 3 : 4 => Total 56 boys

Girl Ratio 4 : 3 => total shall be 84-56=28

-----------------------------------------------------

If so, the no of boy in 5A and 5B shall be

........... 24 : 32

but if so, the no. of girls in 5A and 5B shall be

........... 16 : 12

But if so, the no. of pupil is 5A will only be 24+16.....

The standard answer is 1:1.

Can anyone advise ?

### Re: Q&A - P5 Math

Thanks for highlighting. I take the ratios as they are, and ignore the numbers since both classes have the same number of pupils.alfretztay wrote:I believe that the sentence 'There were 56 boys altogether in both classes.' ought to be deleted in order to arrive at the standard answer. Alternatively, '56' could be changed to 42.numeric wrote:vaun wrote:P5 Math Mid year Exam 2010 - Question 17

There are 42 pupils in each of the classes. primary 5A and primary 5B. The ratio of the no. of boys in 5A to the no. of boys in 5B was 3:4. The ratio of the no. of girls in 5A to the no. of girls in 5B was 4:3. There were 56 boys altogether in both classes. What was the ratio of boys in 5A to the no. of girls in in 5B.

My steps :

......5A (42) : 5B(42)

Boy Ratio 3 : 4 => Total 56 boys

Girl Ratio 4 : 3 => total shall be 84-56=28

-----------------------------------------------------

If so, the no of boy in 5A and 5B shall be

........... 24 : 32

but if so, the no. of girls in 5A and 5B shall be

........... 16 : 12

But if so, the no. of pupil is 5A will only be 24+16.....

The standard answer is 1:1.

Can anyone advise ?

Nothing wrong with your thinking

alfretztay is right.

The question has a contradiction in it.

If there are really 56 boys in both classes and the boy ratio is 3:4, then

5A: 8 boys, 8 boys, 8 boys, 18 girls

5B: 8 boys, 8 boys, 8 boys, 8 boys, 10 girls

18 girls in 5A: 10 girls in 5B won't give a 4:3 ratio.

- Swift Formula
- BlueBelt
**Posts:**246**Joined:**Thu Jun 25,**Total Likes:**2

### Re: Q&A - P5 Math

Wrong. Ratio must always be in same units. No dozens or whatever.vaun wrote:for example, I said the ratio of blue pen in dozen to red pen is 1 dozen of blue pen to 2 red pens, by specifically mention the unit of measurement in the ratio statement. Correct ?

E.G. 7km : 10m = 700 : 1 (<---Notice no units is put in ratio because they cancel each other out)

Ratio = unit-less and whole numbers

56 boys > 42 average.

How to be 4:3 and 3:4 ?

Error in Question.

E.G Pencil A is longer than Pen B. Pen B is longer than Ruler C. Ruler C is longer than Pencil A. Which is the longest?

### Re: Q&A - P5 Math

Thank you all. The discussion helps.

FYI - The question was from Methodist Girl School P5 mid year exam 2010.

FYI - The question was from Methodist Girl School P5 mid year exam 2010.

### Re: Q&A - P5 Math

60% of the number of marbles in box A is equal to 75% of the number of marbles in box B. Box C has half the number of marbles in box A. Box B has 60 marbles more than that in box C.

a) How many marbles are there in box A?

b) How many marbles must be transferred from box B to box C so that box C has 90% of the number of marbles in box A?

a) How many marbles are there in box A?

b) How many marbles must be transferred from box B to box C so that box C has 90% of the number of marbles in box A?

### Re: Q&A - P5 Math

As 60% of 5u is 3u.. We let marbles in box A be 5u

If 3u is 75% or 3/4 of box B

1/4 of B is 1u. Box B will have 4u.

Box C is 2.5u, which is half of Box A.

4u-2.5u = 1.5u = 60

u = 40

a) Box A = 5u = 200 marbles.

b) Box C has has 2.5u = 100 marbles.

90% of Box A = 0.9 X 200 = 180

Box B needs to transfer 80 marbles to Box C in order have 90% of Box A.

If 3u is 75% or 3/4 of box B

1/4 of B is 1u. Box B will have 4u.

Box C is 2.5u, which is half of Box A.

4u-2.5u = 1.5u = 60

u = 40

a) Box A = 5u = 200 marbles.

b) Box C has has 2.5u = 100 marbles.

90% of Box A = 0.9 X 200 = 180

Box B needs to transfer 80 marbles to Box C in order have 90% of Box A.

### Re: Q&A - P5 Math

60% is 3u, not 5u.angel wrote:How do you derive 60% is 5U?

If we let Box A be 5u, then 60% of it will be 3u

### Re: Q&A - P5 Math

angel wrote:60% of the number of marbles in box A is equal to 75% of the number of marbles in box B. Box C has half the number of marbles in box A. Box B has 60 marbles more than that in box C.

a) How many marbles are there in box A?

b) How many marbles must be transferred from box B to box C so that box C has 90% of the number of marbles in box A?