PSLE
Nick had a collection of folders, pens and files. He had 13 folders. 40% of his collection was pens. He had 15 fewer pens than files.

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

(a)
40% = ⁴⁰₁₀₀ = ²₅

Number of pens = 2 u
Number of files = 2 u + 15
Number of folders = 13
Total number of items = 5 u

5 u = 2 u + 2 u + 15 + 13
5 u - 4 u = 28
1 u = 28

Number of pens
= 2 u
= 2 x 28
= 56

(b)
Total number of items at first
= 5 u
= 5 x 28
= 140

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

Number of items after giving away some pens
= 80% x 140
= ⁸⁰₁₀₀ x 140
= 112

Number of files and folders
= 5 u - 2 u
= 3 u
= 3 x 28
= 84

Number of pens left
= 112 - 84
= 28

Percentage of his collection that was pens after that
= ²⁸₁₁₂ x 100%
= 25%

Answer(s): (a) 56; (b) 25%