The answer will be less than 78 kt as mass has reduced.
But to find the answer use the formula original stall speed x SQRT of New Mass
/ Old mass = New Stall speed.
so 78 x SQRT 5000/6850 = 66.6 KTS
Answer 67 KTS
The stall is really dependant on the force of weight? But a change in mass would mean a change in weight.