Jeffrey is going to charge an initial fee of $10 for mowing lawns, plus an hourly rate. He knows that he wants to charge a total of $40 to mow his neighbor's yard, and it will take him 2 hours to complete the job.
At the same rate, how much should he charge another customer for a 4-hour job?
A)
$100
B)
$15
C)
$60
D)
$70