Every
time you go flying you will consider 3 limits....MTOM, MZFM, and MLM.
MTOM and MLM are either structural or performance limits (depending on
conditions), MZFM is a structural limit. Its
up to you to ensure you do not exceed these limits...for any set of conditions,
one of these limits will offer the 'critical value' in terms of payload or
fuel.
For this question you are only concerned with fuel, so MZFM doesn't come into
it....
A table will help
T/O
LAND
LIMIT
4200 3700
DOM 2800 2800
LOAD 400 400
FUEL 1000 500
So max fuel at T/O is 1000kgs and max fuel at landing is 500kgs; now add the
trip fuel to the landing fuel....300+500=800kgs
Therefore, max fuel at T/O dictated by the MLM limit....you can T/O with 800kgs
of fuel (within MTOM limit), you will then land at MLM.