The simple way, least calculator hit (other methods are available) I teach my students is:-
Take-off fuel / Total flight time * Time remaining.
250 / 3.5 * 1.5 = 107 kg.
The question usually says 25% reserve of REMAINING trip fuel, which means the actual amount of reserve fuel required is decreasing as the flight progresses and would be zero on arrival.
As this question didn't ask for a split of trip & reserve the answer is 107 kg (assuming it meant REMAINING). However to split 107 / 1.25 (back work 25%) = 86 kg trip and reserve would be 21 kg which is 25% of 86 kg.
This is a proportion question and the old whiz wheel excels at this problem.
Last edited by RichardH; 18th August 2013 at 14:18.