12. A business manager who needs to make many phone calls has estimated that when she calls a client, the probability that she will reach the client right away is 50%. If she does not reach the client on the first call, the probability that she will reach the client on her second call in the next hour is 20%%. And if she does not reach the client on either the first or second call, the probability that she will reach the client on her third call in the hour after that is 10%%. The probability that the manager will reach her client in three or fewer calls is How did you derive this value