Thirty women were examined in a hospital by a doctor and the number of heartbeats per minute were recorded and summarised as follows. Find the mean heartbeats per minute for these women, choosing a suitable method. \(\begin{array}{|c|c|c|c|c|c|c|c|}\hline \text{Number of heartbeats per minute} & 65 - 68 & 68 - 71 & 71 - 74 & 74 - 77 & 77 - 80 & 80 - 83 & 83 - 86 \\ \hline \text{Number of women} & 2 & 4 & 3 & 8 & 7 & 4 & 2 \\ \hline \end{array}\)