Have got the following question was fine on the first section just not sure on the others, not sure whether the route to go down is to find the probability greater than or equal to 286.5 hours for the d2 component. I tried this just jusing the standard normal distribution functions as in part a however the numbers seem to large....

(c) The life lengths (times to failure, in hours) of two types of electronic component,

*D*1 and

D

2, have distributions N(139*.*6*, *36) and N(145*, *4) respectively.

(i) Which type of component should be used to maximize the probability of running for

at least 148 hours without failure.

(ii) A type *D*1 component is used. When it fails, it is replaced by a type *D*2 component.

What is the probability that the second component is still running after 286.5 hours?

(iii) The two types of device are run in parallel. What is the probability that one device

fails more than 5.4 hours before the other?

Many thanks in advance.