3.74 The time Z in minutes between calls to an elec-trical supply system has the probability density func-tion f(z)=\left\{\begin{array}{ll}

\frac{1}{10} e^{-a / 10}, & 0

0, & \text { elsewhere. }

\end{array}\right. (a) What is the probability that there are no calls withina 20-minute time interval? (b) What is the probability that the first call comes within 10 minutes of opening?