I don't know how to do part (c) and (d)
For (c), you are looking for the probability that , while eliminating the area of the graph up to .
which is
Convert to the Z-values and compute the probability.
Bear in mind that the probability of the bulb failing within the first 850 hours is now zero.
Hence your fraction has in the denominator.
For (d), use the Binomial conversions
where p=0.07, q=0.93, n=1200
Let P(t) be the CDF of the bulb life.
For c you want [P(1350)-P(500)]/(1-P(500)) Imagin you have 1200 bulbs then 1200 P(500) have failed by 500 hours, 1200 P(1350) by 1350 hours, so out of the 1200 (1-P(500)) that survive the first 500 hours 1200 [P(1350)-P(500)] fail in the next 850 hours.
CB