Volume of the prism is given by:
V=(base area)*(height)
thus:
base area=1/2(a+b)*h
=1/2(1.4+0.6)*2
=2 m²
Thus the volume will be:
V=2*1
V=2 m³
Volume after 30 min:
V=2×(0.8)
V=1.6 m³
Rate of emptying the tank is:
(2-1.6)/30=1/ 75 m³/min
time taken to empty the tank:
time=volume/rate
time=2/(1/75)
=150 min