If I toss a coin 3 times what is the probability of it landing on heads at least once?

The probability of getting heads at least once is the same as saying the probability of getting heads: exactly once, exactly twice or exactly three times. This is also 1 minus the probability of getting heads 0 times. It is easiest to find the probability of getting heads 0 times as there is only one way in which this can happen (as can be shown on a probability tree). This way is to on the first, second and third coin flip to get tails. The probability of getting tails on the first coin flip in 0.5, the probability of getting tails on the second and third flip is still 0.5. For this to happen on the first AND the second AND the third flip all we need to remember is that P(A and B) = P(A) x P(B). Meaning in this case that the probability of getting all tails is 0.5 x 0.5 x 0.5 = 0.125. So to find our final answer we just have to take this away from one to get 0.875.