PSLE
Will had green notebooks and black notebooks for sale. He sold 300 green notebooks. 70% of the notebooks sold were black.

1. How many green notebooks and black notebooks did Will sell altogether?
2. He sold 50% of his notebooks. 80% of the notebooks left unsold were green. How many black notebooks did Will have at first?

	Green	Black	Total	Total in percent
Before	11 u	9 u	20 u	100%
Change	- 30% - 3 u	- 70% - 7 u	- 100% - 10 u	- 50%
After	80% 8 u	20% 2 u	100% 10 u	50%

70% = ⁷⁰₁₀₀ = ⁷₁₀

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

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

3 u = 300

1 u = 300 ÷ 3 = 100

Total number of notebooks sold
= 10 u
= 10 x 100
= 1000

(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 green = 80%
Percent of the notebooks left unsold that were black = 100% - 80% = 20%

Number of the notebooks left unsold that were black
= 20% x 10 u
= ²⁰₁₀₀ x 10 u
= 2 u

Number of black notebooks that Will had at first
= 2 u + 7 u
= 9 u
= 9 x 100
= 900

Answer(s): (a) 1000; (b) 900