72. The lifetime X (in hundreds of hours) of a certain type of vacuum tube has a Weibull distribution with parameters a
2 and b
3. Compute the following:

a. E(X) and V(X)

b. P(X  6)

c. P(1.5  X  6)

(This Weibull distribution is suggested as a model for time in service in “On the Assessment of Equipment Reliability:

Trading Data Collection Costs for Precision,” J. Engr.

Manuf., 1991: 105–109.)