Past exam of the mathematics course of the year 1 of the natural sciences course of the University of Cambridge / 2025 / Paper 2 / 12 / e / iii

There are two equally likely balls bearing label eight, one of each colour. The conditional probability that the removed ball was green is therefore $1/2$. By the law of total probability,Both possible first colours must be included because the observed label alone does not determine the colour.