In a nuclear reaction, the mass defect of a nucleus is $5.0 \times 10^{-29}$ kg. The binding energy of the nucleus is approximately [Speed of light $c = 3.0 \times 10^{8}$ m/s]

Using Einstein's mass-energy equivalence, $E = mc^2 = 5.0 \times 10^{-29} \times (3.0 \times 10^{8})^2 = 5.0 \times 10^{-29} \times 9.0 \times 10^{16} = 45 \times 10^{-13} = 4.5 \times 10^{-12}$ J. A common error is using $c$ instead of $c^2$, which gives the wrong answer A.