PSLE
Eric had a collection of folders, pens and staplers. He had 11 folders. 40% of his collection was pens. He had 11 fewer pens than staplers.

1. How many pens did Eric have?
2. Eric 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 staplers = 2 u + 11
Number of folders = 11
Total number of items = 5 u

5 u = 2 u + 2 u + 11 + 11
5 u - 4 u = 22
1 u = 22

Number of pens
= 2 u
= 2 x 22
= 44

(b)
Total number of items at first
= 5 u
= 5 x 22
= 110

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

Number of items after giving away some pens
= 80% x 110
= ⁸⁰₁₀₀ x 110
= 88

Number of staplers and folders
= 5 u - 2 u
= 3 u
= 3 x 22
= 66

Number of pens left
= 88 - 66
= 22

Percentage of his collection that was pens after that
= ²²₈₈ x 100%
= 25%

Answer(s): (a) 44; (b) 25%