For the part B
we have the following information
To get the approximate percentage that remains in service between 48 and 51 months, we will use the formula
and then follow the steps below
Step 1.
We will find the z-score corresponding to both 48 and 51 months
For 48 months
for 51 months
The plot of the distribution is given below
From the plot above, we can observe that the probability is
In terms of percentage, the value is
Thus, we will have