63,000
Explanation
A percent is always over a whole so 15% = .15
60000 • .15 = 9000 yearly
9000/12 = 750 monthly
750 • 4 months = 3000
60000 + 3000 = 63000
If you want to be more precise Change 12 into 365 days and multiply the result of 9000/12 by the average amount of days in four months.
