Mewa Lal borrowed Rs 20000 from his friend Rooplal at 18% per annum simple interest.

SI for Mewa Lal $=\frac{\text { PRT }}{100}$

$=\frac{20,000 \times 18 \times 2}{100}$

$=\operatorname{Rs} 7,200$

Thus, he has to pay Rs 7,200 as interest after borrowing.

CI for Mewa Lal $=\mathrm{A}-\mathrm{P}$

$=20,000\left(1+\frac{18}{100}\right)^{2}-20,000$

$=20,000(1.18)^{2}-20,000$

$=27,848-20,000$

$=\operatorname{Rs} 7,848$

He gained Rs 7,848 as interest after lending.

His gain in the whole transaction $=$ Rs $7,848-$ Rs 7,200

$=$ Rs 648