Answer:
Solid Sounds
Explanation:
As a test, let's say the dance-a-thon lasted 10 hours. Plug in 10 hours to each:
Gabe's: $60/hr x 10 hr = $600
Derek's: $100 + ($40/hr x 10 hr) = $500
Solid Sounds: $175 + ($30/hr x 10 hr) = $475
This shows that Solid Sounds is the cheapest (no matter how many hours the dance lasts)