Area of the base of the reservoir=10.8m*3.75m
the pipe is cuboidal in shape:
cross-section area of the pipe=0.075m*0.045m
volume of water through the pipe in 1sec=cross-sectional area * rate of water
This is the volume filled in the reservoir by the pipe in 1sec
so volume filled by the pipe in 30 mins=0.06075*30*60(1 min =60sec)
This is the total volume in the reservoir after 30 mins.
so height of water level=volume/area of base of the reservoir