This blog is managed by Song Hock Chye, author of Improve Your Thinking Skills in Maths (P1-P3 series), which is published and distributed by EPH.

## Monday, January 21, 2008

### MGS (Paya Lebar) Pri School P6 Math CA1 2006 (Q43)

Last year, the ratio of the number of boys to the number of girls in a chess club was 5 : 3. After 5 boys and 9 girls joined the club this year, the ratio became 5 : 4. How many girls were there in the chess club at first?

Solution

20 units + 20 ----- 15 units + 45
20 units – 15 units ----- 45 – 20
5 units ----- 25
1 unit ----- 25 divided by 5 = 5

(Girls at first) 3 units ----- 5 x 3 = 15

Answer: There were 15 girls at first.

#### 2 comments:

Anonymous said...

Mr Song
Wish to seek your opinion on the use of equivalent ratio to solve this problem. This method may be more suitable for weaker students , but it’s longer
Before After
B:G B:G
5:3 5:4
25:15 30:24

As there is an increase of 5 boys and 9 girls
30-25 =5 boys
24-15 =9 girls
At first there are 15 girls
Will students be penalised if they use this method.

Excel Eduservice said...

Dear Anonymous,

My reply can be found in this link.