One of production steps for a company that makes equipment for the semiconductor industry involves polishing.The polishing machine requires a polishing disk.The amount of time this disk lasts vary from disk to disk.Once a disk wears to a certain point,it must be replaced with a new one.Experience indicates that the time to failure isexponentiallydistributed with a mean of 4.5 hours.The operators are required to log the amount of time each disk lasts.One operator reported the following times for her machine on her 12 hour shift: 2.4 hours, 1.5 hours,2.8 hours,and 3.1 hours(Note,the fifth disk was still okay when she went off shift after 12 hours).

A) For each of the four disks that were replaced,determine the probability that the disk will wear out in that number of hours or less.

HERE I found:

p(x<<1.5)= 0.283469

p(x<<2.8)=0.46325

p(x<<3.1)=0.497866

B)What is the probability that an operator would find 4 successive disks with these hours of performance?

C) Based on your answer to parts A and B, comment on whether you think there is evidence to suggest that the disks currently being used are not meeting the 4.5 hour mean life

P.S I cant still figure out how to solve the b and c))

Would be very nice if someone help!!

Further thanks!!!)))