We will be using the Unitary Method.

For 90 mins, we can use 5 pumps.

For 1 min, we can use (5×90) pumps.

For 30 mins, we can use [(5×90)/30] pumps

= [450/30] pumps

= 15 pumps [Ans.]

Therefore, 15 pumps are needed to fill up the tank in 30 minutes.

Answer:

so first 90/5 =18 so, 1 pump in 18 minutes

so 30 =30/18=1.6666 minutes approx