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 \(4 R T\).