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\begin{align*} speed = \frac{distance}{time} \\ \end{align*} \begin{align*} \therefore distance = speed * time = 120 * 3 = 360km \\ \end{align*} \begin{align*} speed = \frac{360}{5} = 72 \\ \end{align*}

speed = \frac{distance}{time} = \frac{1200}{2 * 60} = 10 m/s \\ \begin{align*} speed = \frac{distance}{time} = \frac{1200}{2 * 60} = 10 m/s \\ \end{align*}

Convert to km/hr

\begin{align*} speed = \left[10 * \frac{18}{5}\right] = 36 km/hr \\ \end{align*}

If car run at speed of 36kmph then x km be the actual distance travelled by car in time t. \begin{align*} time = \frac{distance}{speed} \\ \end{align*} \begin{align*} \therefore t = \frac{x}{36} \\ \end{align*} Now same car if runs at 54kmph then it covers x + 20 km distance in time t - \begin{align*} time = \frac{distance}{speed} \\ \end{align*} \begin{align*} \therefore t = \frac{x+20}{54} \\ \end{align*} Equating both equations - \begin{align*} \therefore \frac{x}{36} = \frac{x+20}{54} \\ \end{align*} \begin{align*} \therefore 3x = 2x + 40 \\ \end{align*} \begin{align*} \therefore x = 40 \\ \end{align*}