PSLE
Ken had a collection of folders, pens and correction tapes. He had 13 folders. 40% of his collection was pens. He had 11 fewer pens than correction tapes.

1. How many pens did Ken have?
2. Ken 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 correction tapes = 2 u + 11
Number of folders = 13
Total number of items = 5 u

5 u = 2 u + 2 u + 11 + 13
5 u - 4 u = 24
1 u = 24

Number of pens
= 2 u
= 2 x 24
= 48

(b)
Total number of items at first
= 5 u
= 5 x 24
= 120

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

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

Number of correction tapes and folders
= 5 u - 2 u
= 3 u
= 3 x 24
= 72

Number of pens left
= 96 - 72
= 24

Percentage of his collection that was pens after that
= ²⁴₉₆ x 100%
= 25%

Answer(s): (a) 48; (b) 25%