Three shooters shoot at the same target, each of them shoots just once. The first one hits the target with a probability of $70\%$, the second one with a probability of $80\%$, and the third one with a probability of $90\%$. What is the probability that the shooters will hit the target
a) at least once?
b) at least twice?
I don't understand well this problem. If each shooter shoots at the target once, how can each of them shoot at it twice?
Do you know how to solve this problem?