A student noted the number of cars passing through a spot on a road for 100 periods each of 3 minutes and summarised it in the table given below. Find the mode of the data: \begin{array}{|c|c|c|c|c|c|c|c|c|} \hline \text{Number of cars} & 0 - 10 & 10 - 20 & 20 - 30 & 30 - 40 & 40 - 50 & 50 - 60 & 60 - 70 & 70 - 80 \\ \hline \text{Frequency} & 7 & 14 & 13 & 12 & 20 & 11 & 15 & 8 \\ \hline \end{array}