First slide
Permutations
Question

The number of ways in which we can get at least 6 successive tails when a coin is tossed 10 times in a row, is

Moderate
Solution

 The sequence of six tails may begin at with the ith toss where i = 1, 2, 3, 4, 5. Thus, the number of ways is

24+123+2(1)22+22(1)2+23(1)=48

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