PSLE
Fred had black notebooks and blue notebooks for sale. He sold 270 black notebooks. 40% of the notebooks sold were blue.

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

	Black	Blue	Total	Total in percent
Before	6.5 u	3.5 u	10 u	100%
Change	- 60% - 3 u	- 40% - 2 u	- 100% - 5 u	- 50%
After	70% 3.5 u	30% 1.5 u	100% 5 u	50%

40% = ⁴⁰₁₀₀ = ²₅

Total number of notebooks sold = 5 u
Number of blue notebooks sold = 2 u

Number of black notebooks sold
= 5 u - 2 u
= 3 u

3 u = 270

1 u = 270 ÷ 3 = 90

Total number of notebooks sold
= 5 u
= 5 x 90
= 450

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

50% of the notebooks that were sold = 5 u
50% of the notebooks that were left unsold = 5 u

Percent of the notebooks left unsold that were black = 70%
Percent of the notebooks left unsold that were blue = 100% - 70% = 30%

Number of the notebooks left unsold that were blue
= 30% x 5 u
= ³⁰₁₀₀ x 5 u
= 1.5 u

Number of blue notebooks that Fred had at first
= 1.5 u + 2 u
= 3.5 u
= 3.5 x 90
= 315

Answer(s): (a) 450; (b) 315