Use mass-energy equivalence

The energy liberated is given by $E = \Delta m \, c^2$

Convert mass defect into SI units

Given mass defect: $\Delta m = 0.4\,\text{g} = 0.4 \times 10^{-3}\,\text{kg} = 4 \times 10^{-4}\,\text{kg}$

Substitute the values

$E = (4 \times 10^{-4})(3 \times 10^8)^2$

$E = (4 \times 10^{-4})(9 \times 10^{16})$

$E = 36 \times 10^{12} = 3.6 \times 10^{13}\,\text{J}$

Convert joules to kWh

We know: $1\,\text{kWh} = 3.6 \times 10^6\,\text{J}$

Therefore, $\text{Energy in kWh} = \frac{3.6 \times 10^{13}}{3.6 \times 10^6} = 10^7\,\text{kWh}$

Compare with the given form

Given: $E = n \times 10^7\,\text{kWh}$

Since $E = 1 \times 10^7\,\text{kWh}$

therefore, $n = 1$

Comparison with stored answer

Stored correct answer = $1$

Our derived answer also is $1$.

Hence, the answer agrees.