Volume of water leaving pipe in t hour should be equal to volume of water in tank with height 7cm.
Hence $$pi r^2 (v imes t) = (L_{tank} imes W_{tank} )H $$
Where r= radius of pipe = 7cm.
v = velocity of water = 5 km/h or $$frac{2500}{18}$$ cm/s.
t= time taken to fill 7 cm.
$$L_{tank}$$ = length of thank = 50m
$$W_{tank}$$ = width of tank = 40m
$$H$$ = filled height = 7cm
After putting values , we will get value of t = 2hr.