Answer:
12.5 hrs
Step-by-step explanation:
The rate at which the mower burns gas is 0.4 gallon / hr.
The greatest volume of gas that the tank can hold is 5 gallons.
If x is the number of hours the mower runs, then:
(0.4 gal/hr)x ≤ 15 gal, or x ≤ (15 gal) / (0.4 gal/hr), or x ≤ 12.5 hrs