Change this from explicit to recursive and explain

Ihave trouble finding the nth term of a linear sequence

Jude is making cement for a driveway. the instructions show the amount of each ingredient to make 1 batch of cement. complete each statement to adjust the ingredients for each new situation if jude uses these instructions.

The plant were you work has a cylindrical oil tank that is 2.5 feet across on the inside and 10 feet high. the depth of the oil in the tank is 2 feet. about how many gallons of oil are in the tank?