We are given that:
Chunk earned : $72.5
Chunk spent: $11.25 for gas and $7.75 for new string.
We need to get the profit. Therefore, we will subtract the amount spent by Chunk from the amount he earned.
Therefore:
Chunk's profit = Amount earned - Amount spent
Chunk's profit = 72.5 - (11.25+7.75)
Chunk's profit = 72.5 - 19
Chunk's profit = $53.5