PSLE
Justin had yellow notebooks and purple notebooks for sale. He sold 120 yellow notebooks. 70% of the notebooks sold were purple.

1. How many yellow notebooks and purple notebooks did Justin sell altogether?
2. He sold 50% of his notebooks. 70% of the notebooks left unsold were yellow. How many purple notebooks did Justin have at first?

	Yellow	Purple	Total	Total in percent
Before	10 u	10 u	20 u	100%
Change	- 30% - 3 u	- 70% - 7 u	- 100% - 10 u	- 50%
After	70% 7 u	30% 3 u	100% 10 u	50%

70% = ⁷⁰₁₀₀ = ⁷₁₀

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

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

3 u = 120

1 u = 120 ÷ 3 = 40

Total number of notebooks sold
= 10 u
= 10 x 40
= 400

(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 yellow = 70%
Percent of the notebooks left unsold that were purple = 100% - 70% = 30%

Number of the notebooks left unsold that were purple
= 30% x 10 u
= ³⁰₁₀₀ x 10 u
= 3 u

Number of purple notebooks that Justin had at first
= 3 u + 7 u
= 10 u
= 10 x 40
= 400

Answer(s): (a) 400; (b) 400