PSLE
Henry had black notebooks and blue notebooks for sale. He sold 420 black notebooks. 30% of the notebooks sold were blue.

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

	Black	Blue	Total	Total in percent
Before	14 u	6 u	20 u	100%
Change	- 70% - 7 u	- 30% - 3 u	- 100% - 10 u	- 50%
After	70% 7 u	30% 3 u	100% 10 u	50%

30% = ³⁰₁₀₀ = ³₁₀

Total number of notebooks sold = 10 u
Number of blue notebooks sold = 3 u

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

7 u = 420

1 u = 420 ÷ 7 = 60

Total number of notebooks sold
= 10 u
= 10 x 60
= 600

(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 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 10 u
= ³⁰₁₀₀ x 10 u
= 3 u

Number of blue notebooks that Henry had at first
= 3 u + 3 u
= 6 u
= 6 x 60
= 360

Answer(s): (a) 600; (b) 360