PSLE
Owen had a collection of punchers, files and post-it pads. He had 11 punchers. 40% of his collection was files. He had 10 fewer files than post-it pads.

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

(a)
40% = ⁴⁰₁₀₀ = ²₅

Number of files = 2 u
Number of post-it pads = 2 u + 10
Number of punchers = 11
Total number of items = 5 u

5 u = 2 u + 2 u + 10 + 11
5 u - 4 u = 21
1 u = 21

Number of files
= 2 u
= 2 x 21
= 42

(b)
Total number of items at first
= 5 u
= 5 x 21
= 105

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

Number of items after giving away some files
= 80% x 105
= ⁸⁰₁₀₀ x 105
= 84

Number of post-it pads and punchers
= 5 u - 2 u
= 3 u
= 3 x 21
= 63

Number of files left
= 84 - 63
= 21

Percentage of his collection that was files after that
= ²¹₈₄ x 100%
= 25%

Answer(s): (a) 42; (b) 25%