Each week, carrie works 40 hours. she spends 15% of her time at the register, 30% of her time greeting customers, and the remainder of the time performing maintenance duties. what percentage of carrie's time is spent performing maintenance duties?

First, you would need to create an equation for this problem. 
100 - (15% + 30%)

Now, all you have to do is solve it. Remeber, using the order of operations (PEMDAS), you would start in the parenthesis. 
100% - (15% + 30%)
100% - 45% = 55%

Carrie spent 55% or 22 hours of her time performing maintenance duties.

I hope this helps!