What is the probability that a couple with 3 children will have at least 1 girl?

The easiest way of calculating this is to find the probability that all three are boys, as this is the only arrangement that does not fit the criteria. Then work out the answer by taking this away from 1. Probability that all three are boys = 1/2 x 1/2 x 1/2 = 1/8. probability of there being at least one girl is 1 - 1/8 = 7/8 or 87.5%