The apparent weight of man inside a lift moving up with certain acceleration is 900N. When the lift is coming down with the same acceleration apparent weight is found to be 300N. The mass of the man is (g = 10 ms^–2)

Apparent weight when the lift is moving with an acceleration a is given by 

$\mathrm{W}=\mathrm{m}(\mathrm{g}+\mathrm{a})=900\mathrm{N}$

And similarly when the lift is moving down with an acceleration, the apparent weight is given by $\mathrm{W}=\mathrm{m}(\mathrm{g}-\mathrm{a})=300\mathrm{N}$

On solving we get mass of the man = 60kg