Let, initial momentum of body $$({p_i}) = p$$

$$\therefore$$ Final momentum $$({p_f}) = {p_i} + 20\% $$ of $${p_i}$$

$$ = p + 0.2~p$$

$$ = 1.2~p$$

We know,

Kinetic energy $$(E) = {{{p^2}} \over {2m}}$$

$$\therefore$$ $${E_i} = {{{p^2}} \over {2m}}$$

and $${E_f} = {{{{(1.2p)}^2}} \over {2m}} = {{1.44\,{p^2}} \over {2m}}$$

$$\therefore$$ % Change in kinetic energy

$$ = {{{E_f} - {E_i}} \over {{E_i}}} \times 100$$

$$ = {{{{1.44\,{p^2}} \over {2m}} - {{{p^2}} \over {2m}}} \over {{{{p^2}} \over {2m}}}} \times 100$$

$$ = {{{{{p^2}} \over {2m}}(1.44 - 1)} \over {{{{p^2}} \over {2m}}}} \times 100 = 0.44 \times 100 = 44$$