Question
A pair of unbiased dice is rolled together till a sum of either 5 or 7 is obtained. The probability that 5 comes before 7 is

2/5

3/5

4/5

None of these
medium
Solution
2/5
Let A denote the event that a sum of 5 occurs, B the event that a sum of 7 occurs and C the event that neither a sum of 5 nor a sum of 7 occurs. We have
Thus,
P (A occurs before B)
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