A body of mass 1 kg is thrown upwards with a velocity 20 ${\mathrm{ms}}^{-1}$. It momentarily comes to rest after attaining a height of 18 m. How much energy is lost due to air friction? (g = $10 {\mathrm{ms}}^{-2}$)

The energy lost due to air friction is equal to difference of initial kinetic energy and final potential energy. 

Initially body possesses only kinetic energy and after attaining a height the kinetic energy is zero.
Therefore, loss of energy = KE - PE

$= \frac{1}{2}{\mathrm{mv}}^2-\mathrm{mgh}$

= $\frac{1}{2}\times 1\times 400-1\times 18\times 10$

= 200-180 = 20 J