PSLE
Ben had black notebooks and blue notebooks for sale. He sold 220 black notebooks. 60% of the notebooks sold were blue.

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

	Black	Blue	Total	Total in percent
Before	5.75 u	4.25 u	10 u	100%
Change	- 40% - 2 u	- 60% - 3 u	- 100% - 5 u	- 50%
After	75% 3.75 u	25% 1.25 u	100% 5 u	50%

60% = ⁶⁰₁₀₀ = ³₅

Total number of notebooks sold = 5 u
Number of blue notebooks sold = 3 u

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

2 u = 220

1 u = 220 ÷ 2 = 110

Total number of notebooks sold
= 5 u
= 5 x 110
= 550

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

Number of the notebooks left unsold that were blue
= 25% x 5 u
= ²⁵₁₀₀ x 5 u
= 1.25 u

Number of blue notebooks that Ben had at first
= 1.25 u + 3 u
= 4.25 u
= 4.25 x 110
= 467.5

Answer(s): (a) 550; (b) 467.5