Hi, can anyone please help me with this question? I really don't know how to do it. Thanks a lot. I'm a bit confused with the situation.

The time to failure of a new type of light bulb is thought to have a exponential distribution.

(a)The reliability of a component is defined as the probability that the component will not have failed by a specified time. If the reliability of this type of light bulb at 10.5 weeks if 0.9, find the reliability at 10 weeks.

(b)One hundred light bulbs of this type are put in a new shop. All light bulbs that have failed are replaced at 20 week intervals and none are replaced at other times. If R is the number of light bulbs that have to be replaced at the end of the first interval, find the mean and variance of R. Explain why this result holds for any such interval, and not just the first.

For Part (a), I did try to use fomula, so I tried to solve $\displaystyle 0.9=e^{-\alpha*10.5}$, then $\displaystyle \alpha=0.01003$, then I did $\displaystyle Pr(T=10)=e^-0.01003*10=0.9045$, so should my answer be 0.9045? Did I use right way?

I really don't know how to do Part (b), please help me with it. Thanks a lot.