PSLE
Will had yellow staplers and black staplers for sale. He sold 210 yellow staplers. 30% of the staplers sold were black.

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

	Yellow	Black	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 staplers sold = 10 u
Number of black staplers sold = 3 u

Number of yellow staplers sold
= 10 u - 3 u
= 7 u

7 u = 210

1 u = 210 ÷ 7 = 30

Total number of staplers sold
= 10 u
= 10 x 30
= 300

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

50% of the staplers that were sold = 10 u
50% of the staplers that were left unsold = 10 u

Percent of the staplers left unsold that were yellow = 70%
Percent of the staplers left unsold that were black = 100% - 70% = 30%

Number of the staplers left unsold that were black
= 30% x 10 u
= ³⁰₁₀₀ x 10 u
= 3 u

Number of black staplers that Will had at first
= 3 u + 3 u
= 6 u
= 6 x 30
= 180

Answer(s): (a) 300; (b) 180