Assuming that all 5 months have 30 working days.
Let us assume that you need to do 'x' hours per month to get the extra money for vacation.
Given that overtime wage is $11.25 per hour, the wage (in $) for monthly overtime is obtained as,

Then the total amount (TA) earned in 5 months of overtime work is calculated as,

Given that the 15% of this amount earned goes for the tax, so the net amount earned is given by,

This net amount must be sufficient to cover the total cost of vacation $2245,

Thus, you have to work 47 hours of overtime monthly to cover the cost of the vacation in 5 months.