PSLE
Gwen bought some chocolate bars and gave half of it to Eric. Eric bought some sweets and gave half of it to Gwen. Gwen ate 3 sweets and Eric ate 21 chocolate bars. The ratio of Gwen's sweets to chocolate bars became 1 : 5 and the ratio of Eric's sweets to chocolate bars became 1 : 3. How many chocolate bars did Gwen buy?

	Gwen		Eric
	Sweets	Chocolate bars	Sweets	Chocolate bars
Before	1 u + 3	5 u	1 p	3 p + 21
Change	- 3			- 21
After	1 u	5 u	1 p	3 p

After Gwen gave half of the chocolate bars to Eric and Eric gave half of his sweets to Gwen, Eric and Gwen had same number of sweets and chocolate bars before each ate some.

5 u = 3 p + 21 
5 u - 21 = 3 p --- (1)

1 u + 3 = 1 p --- (2)

Make p the same.

(2)_x3
3 u + 9 = 3 p --- (3)

(1) = (3)
5 u - 21 = 3 u + 9 
5 u - 3 u = 9 + 21
2 u = 30

1 u = 30 ÷ 2 = 15

Number of chocolate bars that Gwen bought
= 2 x 5 u
= 10 u
= 10 x 15
= 150

Answer(s): 150