Niki uses 15 litres of water every minute when she has a shower.

She uses 100 litres of water when she has a bath.

How many litres of water does she save by having a 3 1/2 minute shower instead of a bath?

15 litres per minute times three minutes is 45 litres for three minutes, and half a minute's worth of water in the shower is 7 1/2 (half of 15). 45+7 1/2 = 52 1/2 litres of water. The difference between 100 and 52 1/2 is 47 1/2, which is our final answer.