Area of the base of the reservoir=10.8m*3.75m

=40.5sq.m

the pipe is cuboidal in shape:

cross-section area of the pipe=0.075m*0.045m

=.0.003375sq.m

volume of water through the pipe in 1sec=cross-sectional area * rate of water

=0.003375sq.m*18

=0.06075cu.m

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)

=109.35 cu.m

This is the total volume in the reservoir after 30 mins.

so height of water level=volume/area of base of the reservoir

=109.35/33.75

=3.24 m