Answer: 5/12
Step-by-step explanation:
Chances of getting box A or B = 1/2
Chances of getting dark choco in Box A:
P = 5/15 = 1/3
Chances of getting dark choco in Box B:
P = 12/24 = 1/2
The probability of getting a dark choco from a random box is:
P = 1/2(1/3 + 1/2) = 1/2(2/6 + 3/6) = 1/2(5/6) = 5/12