10

cubic metre

The maximum water fee is 1.6* 10= 16.

11-15m3

Pay the water fee at most.

16+5*2=26

It cost 33.5 yuan

The cost above 15 is 33.5-26=7.5. The cost above 15 is 2.5 cubic meters, so the total cost is 17.5 cubic meters.