ANSWER:
✔ 1 over 3 ⋅ 12 hours
EXPLANATION:
1/12 of a tank in 20 minutes.
Thus we know in an hour, an oil company can fill 3/12 of a tank (60 minutes in an hour).
3 x 4 = 12, and 12/12 = 1(a whole tank),
so we multiply 1 (hour) by 4.
Our answer is that it takes 4 hours to fill a tank.