Total number of balls = 25
P(green ball) = 3/5
Number of green balls = (3/5) × 25 = 15
Using complementary events: P(yellow) = 1 − 3/5 = 2/5
Number of yellow balls = (2/5) × 25 = 10
Source: Chapter 14, Section 14.1
The examiner expects you to first find the number of green balls (or directly use the complement), then subtract from 25. Both methods are acceptable, but showing the complementary probability step earns full marks. Key formula used: Number of favourable outcomes = P(E) × Total outcomes.