Work out your fuel flow using the latest ACTUAL information .... then use that fuel flow to predict your FUEL at the required next point - in this case MIKE.

So...

ACTUAL TIME BETA (1:22) to GAMMA (1:47) = 25 mins

ACTUAL FUEL BETA (2760 kg) to GAMMA (2360 kg) = 400 kg

FUEL FLOW BETA to GAMMA = 400 / 25 = 16 kg / min

FUEL USED GAMMA to MIKE = 68 x 16 = 1088 kg

SO, FUEL AT MIKE = 2360 - 1088 = 1272 kg