PSLE
Oliver had a collection of punchers, files and glues. He had 28 punchers. 30% of his collection was files. He had 20 fewer files than glues.

1. How many files did Oliver have?
2. Oliver gave away some files. His total collection decreased by 20%. What percentage of his collection was files after that?

(a)
30% = ³⁰₁₀₀ = ³₁₀

Number of files = 3 u
Number of glues = 3 u + 20
Number of punchers = 28
Total number of items = 10 u

10 u = 3 u + 3 u + 20 + 28
10 u - 6 u = 48
4 u = 48

1 u = 48 ÷ 4 = 12

Number of files
= 3 u
= 3 x 12
= 36

(b)
Total number of items at first
= 10 u
= 10 x 12
= 120

Total collection in the end in percent
= 100% - 20%
= 80%

Number of items after giving away some files
= 80% x 120
= ⁸⁰₁₀₀ x 120
= 96

Number of glues and punchers
= 10 u - 3 u
= 7 u
= 7 x 12
= 84

Number of files left
= 96 - 84
= 12

Percentage of his collection that was files after that
= ¹²₉₆ x 100%
= 12.5%

Answer(s): (a) 36; (b) 12.5%