I think he means "show your working". Tsk, haven't you just finished your A-levels? Shouldn't that be ingrained still? Don't worry, the alcohol will scrub it from your brain - you don't have to fight it

Anyway, my working (although I didn't follow it through to a conclusion) went something like:
In the morning there must be 5n coconuts where n is integer.
If there are x coconuts left after each person has taken his share then there must be 5x/4 + 1 before that person takes his share.
So:
[1] 25n/4 + 1 is integer
[2] 125n/16 + 5/4 + 1 is integer
[3] 625n/64 + 25/16 + 5/4 + 1 is integer
[4] 3125n/256 + 125/64 + 25/16 + 5/4 + 1 is integer
[5] 15625n/1024 + 625/256 + 125/64 + 25/16 + 5/4 + 1 is integer

This is as far as i got before I went home last night. It's just a case of finding the smallest n that satisfies the above, then working out the answer to eq [5]