Answer
It would take 80 days for the man to lose 5 kg.
Work Step by Step
Total energy consumption for a 68-kg man:
$E_c=860+600+540+639+72\times20=4079\ Cal/day$
For a 75-kg man:
$E_c=4498.90\ Cal/day$
Since the intake is 4000 Cal/day,
$\Delta E=498.9\ Cal/day$
$\Delta t=\Delta m\times e \div \Delta E$
With $e=33.1 MJ/kg, \Delta m = 5\ kg$
So
$\Delta t=79.2\ days$
It would take 80 days for the man to lose 5 kg.
