A train runs for 4 hours at a speed of 75 km per hour and the next 3 hours at a speed of 55 per hour. Find the average speed of the train for the 7 hours journey.

Question

$\begin{array}{11} (a)\; 67 \\ (b)\; 65.56 \\ (c)\; 66.42 \\ (d)\; 67.34 \end{array}$