Lonely Guy Repair Service recently performed repair services for a customer that totaled $400. Somehow the bill was lost and the company accountant was trying to recreate the bill from memory. This is what was remembered:

Total bill $600
Labor profit margin $10
Materials profit margin 20%
Total labor charges $390
Cost of materials used $120
Total hourly cost $22.50

What was the material loading charge?

a) 43.8%
b) 61.3%
c) 75%
d) 37.5%

Answer: c) 75%