One pipe can fill a tank with oil in 3 hours. A second smaller pipe can fill the same tank in 7 hours. If both pipes were used at the same time, how long would they take to fill the tank?

2.1 hours.

Step-by-step explanation:

Given , Time taken by the first pipe to fill  a tank = 3 hours

Time taken by the second pipe to fill the tank = 7 hours

If both pipes were used at the same time,  then the time taken (t) by both of them to fill the tank ( together) will be :-

Therefore, time taken to fill the tank = 2.1 hours.