Start from the MLM ... this is key:
Max fuel you can land with = MLM - Actual ZFM : 200kgs (A)
Extra fuel available with take off fuel of 300kg = 300 - 190 (trip fuel) = 110kgs (B)
Extra fuel that can still be uplifted: (A) - (B) = 90kgs
The above is a matter of logic ... cheers.