PSLE
Lee had blue notebooks and yellow notebooks for sale. He sold 90 blue notebooks. 70% of the notebooks sold were yellow.

1. How many blue notebooks and yellow notebooks did Lee sell altogether?
2. He sold 50% of his notebooks. 75% of the notebooks left unsold were blue. How many yellow notebooks did Lee have at first?

	Blue	Yellow	Total	Total in percent
Before	10.5 u	9.5 u	20 u	100%
Change	- 30% - 3 u	- 70% - 7 u	- 100% - 10 u	- 50%
After	75% 7.5 u	25% 2.5 u	100% 10 u	50%

70% = ⁷⁰₁₀₀ = ⁷₁₀

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

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

3 u = 90

1 u = 90 ÷ 3 = 30

Total number of notebooks sold
= 10 u
= 10 x 30
= 300

(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 blue = 75%
Percent of the notebooks left unsold that were yellow = 100% - 75% = 25%

Number of the notebooks left unsold that were yellow
= 25% x 10 u
= ²⁵₁₀₀ x 10 u
= 2.5 u

Number of yellow notebooks that Lee had at first
= 2.5 u + 7 u
= 9.5 u
= 9.5 x 30
= 285

Answer(s): (a) 300; (b) 285