Given:-
Maria recives 2.5% commission when he sells $100 worth of t-shirts.
To find:-
What is the commission if he solds $400 worth of t-shirts.
Now to find the commission for $100. we get,

So he gets $2.5 when he sold $100.
The commission for $400 is,

So he gets $10 when he sold $400.
So the required solution is $10.