A car of mass 900 kg moves at 12 m/s. Later it moves at 24 m/s…

Step 1. At $12\ \text{m/s}$: $\tfrac{1}{2} \times 900\ \text{kg} \times (12\ \text{m/s})^2 = 64\,800\ \text{J}$.

Step 2. At $24\ \text{m/s}$: $\tfrac{1}{2} \times 900\ \text{kg} \times (24\ \text{m/s})^2 = 259\,200\ \text{J}$.

Step 3. $\dfrac{259\,200\ \text{J}}{64\,800\ \text{J}} = 4$.