20cm=0.2m
1.6m-0.2m=1.4m
radius=2.5, area of circle=19.635m^2
19.635m^2x1.4m=27.489m^3
1litre=0.001m^3
1.35litres=0.00135m^3
27.489^3/0.00135m^3=20362s
60s=1min
20362s/60s=339.37min
=5 hours and 39 minutes
(intermediate steps are left in 5s.f.)
hope this helps