24. The route used by a certain motorist in commuting to work contains two intersections with traf c signals.

The probability that he must stop at the rst signal is .4, the analogous probability for the second signal is .5, and the probability that he must stop at at least one of the two signals is .6. What is the probability that he must stop

a. At both signals?

b. At the rst signal but not at the second one?

c. At exactly one signal?