This is given as 1. P ( T ) {\displaystyle P(T)} , or the probability of a (randomly selected) student wearing trousers regardless of any other information. Since P ( T ) = P ( T | G ) P ( G ) + P ( T | B ) P ( B ) {\displaystyle P(T)=P(T|G)P(G)+P(T|B)P(B)} (via the law of total probability), this is P ( T ) = 0.5 × 0.4 + 1 × 0.6 = 0.8 {\displaystyle P(T)=0.5\times 0.4+1\times 0.6=0.8} .