\(S=4.231 g ^{\circ} C\)

\(v=3 \times 10^9 \,Hz\)

\(m=400 \,g\)

Energy of one photon \(=h v\)

Heat needed for \(20^{\circ} C\) temp. change \(=m s \Delta T=400 \times 4.2 \times 20 \,J\)

\(\therefore\) Number of photons \(=\frac{400 \times 4.2 \times 20}{h v}=\frac{400 \times 4.2 \times 20}{6.63 \times 10^{-34} \times 3 \times 10^9}=1689 \times 10^{25}=1.69 \times 10^{28}\)