How many tosses of a fair coin are needed so that the probability of getting atleast one head is 0.875 ?
Similar to your other question:
1 - (Probability not getting any heads) = 0.875
So, you want the probability of not getting any heads to equal 0.125.
For one toss, the probability of not getting a head = 0.5.
For two tosses the probability of not getting a head = 0.5 * 0.5 = 0.25
etc.