Answer:
$80
Explanation:
First I'll ask (cause I'm not an expert at english)(and I'm a child)
Additional fee for maintenance is a thing you pay every month right?
In that case,
Monthly fee per month (m): 11
Additional fee per month (a): 9
Total fee per month (t): [m+a] or [11+9]
Total fee for 4 months (x): (11+9)x4=80
Customers will have to pay $80 after 4 months.