Use the latest ACTUAL DATA (Beta - Gamma) to calculate FUEL FLOW ....

Time (B-G) = 1:42 - 1:18 = 24 mins
Fuel (B-G) = 3180 - 2700 = 480 kg

FUEL FLOW B-G = (480 / 24) = 20 kg / min

FUEL USED G - M = FUEL FLOW x TIME = 20 x 40 = 800 kg

FUEL ON ARRIVAL AT M = 2700 - 800 = 1900 kg