Heat supplied $=\Delta U=U_{\text {final }}-U_{\text {initial }}$

$U_{\text {initial }}=\frac{5}{2} \times 20 \times R T, U_{\text {final }}=\frac{5}{2} \times(20-8) R T+\frac{3}{2} \times(2 \times 8) R T$

$\Delta U=\frac{1}{2} \times 8 \times R T$

$=4 R T$

$\Rightarrow$ Heat energy given is $4 R T$.