$ K=\frac{1}{2}mV^2 \\ K_1 = \frac{10000 \times (100)^2}{2}=5 \times 10^7J = 5 \times 10^4kJ \\ K_2 = \frac{10000 \times (120)^2}{2}=7/2 \times 10^7J = 7/2 \times 10^4kJ \\ K_2 - K_1 = 7/2 \times 10^4 - 5 \times 10^4 = 2/2 \times 10^4kJ $