PSLE
David had red notebooks and pink notebooks for sale. He sold 360 red notebooks. 70% of the notebooks sold were pink.

1. How many red notebooks and pink notebooks did David sell altogether?
2. He sold 50% of his notebooks. 60% of the notebooks left unsold were red. How many pink notebooks did David have at first?

	Red	Pink	Total	Total in percent
Before	9 u	11 u	20 u	100%
Change	- 30% - 3 u	- 70% - 7 u	- 100% - 10 u	- 50%
After	60% 6 u	40% 4 u	100% 10 u	50%

70% = ⁷⁰₁₀₀ = ⁷₁₀

Total number of notebooks sold = 10 u
Number of pink notebooks sold = 7 u

Number of red notebooks sold
= 10 u - 7 u
= 3 u

3 u = 360

1 u = 360 ÷ 3 = 120

Total number of notebooks sold
= 10 u
= 10 x 120
= 1200

(b)
Percent of the notebooks left unsold
= 100% - 50%
= 50%

50% of the notebooks that were sold = 10 u
50% of the notebooks that were left unsold = 10 u

Percent of the notebooks left unsold that were red = 60%
Percent of the notebooks left unsold that were pink = 100% - 60% = 40%

Number of the notebooks left unsold that were pink
= 40% x 10 u
= ⁴⁰₁₀₀ x 10 u
= 4 u

Number of pink notebooks that David had at first
= 4 u + 7 u
= 11 u
= 11 x 120
= 1320

Answer(s): (a) 1200; (b) 1320