Question
How many times must a fair coin be tossed so that the probability of getting at least one head is more than 80%?
Solution
Let p denotes the probability of getting heads.
Let q denotes the probability of getting tails.
Suppose the coin is tossed n times.
Let X denote the number of times of getting heads in n trails. 
So the fair coin should be tossed for 3 or more times for getting the required probability.
