Hello Yulien,
You're told that:
- Darrin orders 24.5 square feet of granite
- He paid a total of $428.75
To find how much he paid per square foot, you would divide the total money spent by the total number of square feet ------> 428.75/24.5
You can type this in your calculator. The answer you get will be the cost per square foot of granite.
Cheers!