a consulting engineer's time is billed at $60 per hour, and her assistant's is builled at $20 per hour. A customer received a bill for $580 for a certain job. If the assistant worked 5 hours less than the engineer, how much time did each bill on the job?

Answer 1

if the engineer worked x hours, the bill is

60x + 20(x-5) = 580
x = 8.5

so, the asst worked 3.5 hours

check:
60*8.5 = 510
20*3.5 = 70