PSLE
Luke had black notebooks and orange notebooks for sale. He sold 770 black notebooks. 30% of the notebooks sold were orange.

1. How many black notebooks and orange notebooks did Luke sell altogether?
2. He sold 50% of his notebooks. 60% of the notebooks left unsold were black. How many orange notebooks did Luke have at first?

	Black	Orange	Total	Total in percent
Before	13 u	7 u	20 u	100%
Change	- 70% - 7 u	- 30% - 3 u	- 100% - 10 u	- 50%
After	60% 6 u	40% 4 u	100% 10 u	50%

30% = ³⁰₁₀₀ = ³₁₀

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

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

7 u = 770

1 u = 770 ÷ 7 = 110

Total number of notebooks sold
= 10 u
= 10 x 110
= 1100

(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 = 60%
Percent of the notebooks left unsold that were orange = 100% - 60% = 40%

Number of the notebooks left unsold that were orange
= 40% x 10 u
= ⁴⁰₁₀₀ x 10 u
= 4 u

Number of orange notebooks that Luke had at first
= 4 u + 3 u
= 7 u
= 7 x 110
= 770

Answer(s): (a) 1100; (b) 770