PSLE
Dylan had gold notebooks and black notebooks for sale. He sold 180 gold notebooks. 60% of the notebooks sold were black.

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

	Gold	Black	Total	Total in percent
Before	5.5 u	4.5 u	10 u	100%
Change	- 40% - 2 u	- 60% - 3 u	- 100% - 5 u	- 50%
After	70% 3.5 u	30% 1.5 u	100% 5 u	50%

60% = ⁶⁰₁₀₀ = ³₅

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

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

2 u = 180

1 u = 180 ÷ 2 = 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 gold = 70%
Percent of the notebooks left unsold that were black = 100% - 70% = 30%

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

Number of black notebooks that Dylan had at first
= 1.5 u + 3 u
= 4.5 u
= 4.5 x 90
= 405

Answer(s): (a) 450; (b) 405