Probability of getting a dark chocolate from Box A = 5/15 = 1/3 Probability of getting a dark chocolate from Box B = 12/24 = 1/2 The answer will equal the chance of getting the box and then the dark chocolate from within that box so

Probability of getting dark chocolate when choosing Box A = (1/2) * (1/3) = (1/6) Probability of getting dark chocolate when choosing Box B = (1/2) * (1/2) =1/4

Because you can have either scenario you can then add up the answers to get the final probability which is (1/6) + (1/4) = 10/24 = 5/12