Answer:
Martha ran at 12 miles per hour for x amount of time
Step-by-step explanation:
To solve for x, simply rearrange the problem:

80% = 6 in
100%=?
100/x 80/6 (proportional relationship)
then cross multiply
600 = 80x
600/80= 7.5
x=7.5
The correct answer is D. 40