How did you get on?
Taking out the relevant information from the text gives you…..
Distance 66.6nm - tidal advantage 2.6nm = 64nm
Speed 12 kts . Journey time 64/12 = 5hrs 20 mins
5 hrs @ 20 lit per hr = 100 lit
20 lit per hr = 20/60 = 0.333 per minute
20 mins = 0.333 x 20 = 6.66 lit fuel
Total fuel for journey = 106.66 lit.
If we were doing a journey that was a return trip, we would use the rule of thirds - 1 third out, 1third back ,1 third in reserve. Even though we are staying, we will use the same principal. Divide the anticipated fuel used by 2 then multiply by 3 to give the fuel calculation for the passage plan.
106.66/2 = 53.33 x 3 159.99 so we will round up and calculate for 160 lit of fuel.
If you managed to get close to that - awesome.