Mean

The masses of 50 pears are grouped as follows:
40<$x$≤50: 5 pears
50<$x$≤60: 15 pears
60<$x$≤70: 20 pears
70<$x$≤80: 10 pears
Using midpoints 45, 55, 65 and 75, find the mean mass of the pears.