PSLE
Ken had a collection of glues, pens and foolscaps. He had 61 glues. 30% of his collection was pens. He had 11 fewer pens than foolscaps.

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)
30% = ³⁰₁₀₀ = ³₁₀

Number of pens = 3 u
Number of foolscaps = 3 u + 11
Number of glues = 61
Total number of items = 10 u

10 u = 3 u + 3 u + 11 + 61
10 u - 6 u = 72
4 u = 72

1 u = 72 ÷ 4 = 18

Number of pens
= 3 u
= 3 x 18
= 54

(b)
Total number of items at first
= 10 u
= 10 x 18
= 180

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

Number of items after giving away some pens
= 80% x 180
= ⁸⁰₁₀₀ x 180
= 144

Number of foolscaps and glues
= 10 u - 3 u
= 7 u
= 7 x 18
= 126

Number of pens left
= 144 - 126
= 18

Percentage of his collection that was pens after that
= ¹⁸₁₄₄ x 100%
= 12.5%

Answer(s): (a) 54; (b) 12.5%