Hello from MrBillDoesMath!
Answer:
(4/5)d + 6/5
Discussion:
2/5(3d+3) =
2(2d+3)/5 =
(4d + 6)/ 5 =
(4/5)d + 6/5
Thank you,
MrB
Answer:
36.58% probability that one of the devices fail
Step-by-step explanation:
For each device, there are only two possible outcomes. Either it fails, or it does not fail. The probability of a device failling is independent of other devices. So we use the binomial probability distribution to solve this question.
Binomial probability distribution
The binomial probability is the probability of exactly x successes on n repeated trials, and X can only have two outcomes.

In which
is the number of different combinations of x objects from a set of n elements, given by the following formula.

And p is the probability of X happening.
A total of 15 devices will be used.
This means that 
Assume that each device has a probability of 0.05 of failure during the course of the monitoring period.
This means that 
What is the probability that one of the devices fail?
This is 


36.58% probability that one of the devices fail
You do 9x27=243 then take 2x31=62 add 243+62=305 then take 305 and subtract it from 28 305-28=277
so if i did it right you should get n=277