Question

A pair of fair dice is rolled together till a sum of either 5 or 7 is obtained. Then, the probability that 5 comes before 7 is ....

Answer 1

5 can be thrown in 4 ways and 7 can be thrown in 6 ways, hence number of ways of throwing neither 5 nor 7 is 

36 - (4 + 6) = 26 

.'. Probability of throwing a five in a single throw with a pair of dice is = 4/36 = 1/9 and probability of throwing neither 5  nor 7 is = 26/36 = 13/18.

Hence, required probability