Answer: 15 pumps
More the number of pumps lesser is the time taken to fill the tank
Number of pumps ∝ 1/Time
n = k / t
[where k = constant]
In 1st case
5 = k / (1.5)
k = 7.5
In second case
n = k / t
= 7.5 / 0.5
= 15
∴ 15 pumps are used to fill the empty tank in half an hour.