A Rancher Has a 5000-Gallon Water Tank That He Uses

A rancher has a 5000-gallon water tank that he uses to water his cattle. He uses 250 gallons per day for the cows.
-Find the zero of this function. Round to the nearest whole day.

A) 20 days
B) 22 days
C) 21 days
D) 23 days
E) none of these