Thirty women were examined in a hospital by a doctor and the number of heart beats per minute recorded and summarised as follows. Find the mean heart beats per minute for these women, choosing a suitable method. No. of heat     65-68   68-71   71-74   74-77   77-80   80-83   83-86 beats (per min.) No. of women:  2           4          3           8         7            4          2