Availability is given by $\frac{Mean Time To Failure}{MeanTimeToFailure+MeanTimeToRepair}$

And $Mean Time Between Failure = Mean Time To Failure + Mean Time To Repair$

Before Enhancement:

Given 90 = $\frac{MTTF}{200}$

MTTF = 180 days

MTTR = MTBF – MTTF

MTTR = 20 days

After Enhancement :

MTTR increases by 5 days. So MTTR = 25 days

95 % = $\frac{MTTF}{MTTF+25}$

MTTF = 475

MTBF = 475 + 25

MTBF = 500 exactly

Option (c)

And $Mean Time Between Failure = Mean Time To Failure + Mean Time To Repair$

Before Enhancement:

Given 90 = $\frac{MTTF}{200}$

MTTF = 180 days

MTTR = MTBF – MTTF

MTTR = 20 days

After Enhancement :

MTTR increases by 5 days. So MTTR = 25 days

95 % = $\frac{MTTF}{MTTF+25}$

MTTF = 475

MTBF = 475 + 25

MTBF = 500 exactly

Option (c)