The time, t, take to buy fuel at a petrol station varies directly as the number of vehicles, v, in the queue and inversely as the number of pumps, p, available in the station. In a station, with 5 pumps, it took 10 minutes to fuel 20 vehicles. Find the number of pumps required to fuel 36 vehicles in 15 minutes.

2 pumps

4 pumps

6 pumps

8 pumps

Correct Answer

Incorrect Answer

How to get the right answer:

v = number of vehicles in the queue. p = number of pumps in the station. From the joint variation, we know that t = kv / p where when t = 10 and v = 20, p = 5 10 = k x 20 / 5 10 x 5 = k x 20 k = 10 x 5 / 20 = 5 / 2 The relation between t, p and v is therefore t = 5v/2p When v = 36 and t = 15 minutes, 15 = 5 x 36 / 2p P = 5 x 36 / 2 x 15 = 6 pumps