After computing the probability for success after 4 flips (4th row of Pascal's triangle: 6/16, not 1/4 as somebody else indicated), the entries representing success (C(4,2) in this case) are replaced with zeros because the process stops with them. This affects the numbers in the following rows. Then it's on to the 6th row, which is now 1 6 9 8 9 6 1. The probability for success after 6 flips is (1 - 6/16)*(9+8+9)/(1+6+9+8+9+6+1). Continue for as long as you like.