Can you please help me solve this equation

Ok, my friends and I are playing a math game. We give each other equations. If we get it right we get ten dollars. My friend gave me this equation:

A computer store buys a computer program for $24 and sells it for $91.20. What is the percent of increase in the price?

Ok, I don't get it! Can you help me? I want that TEN DOLLARS!

280%

% increase
= ( 91.20 - 24) / 24 x 100%
= 67.20 /24 x 100%
= 280%