A pair of dice is rolled until a sum of either 6 or 9 appears. Find the probability that a sum of 6occurs first.
There are 5 ways that a sum of six can occur [ 1,5 2,4 3,3 4,2 5,1 ].
There are 4 ways that a sum of nine can occur [ 6,3 5,4 4,5 3,6 ].
The probability that a sum of six occurs first is 5/9.