Find the mean of the data summarized in the frequency table

Find the mean of the data summarized in the frequency table below. Find the mean speed:

Speed (mph) Cars

30-39------------ 3

40-49----------- 18

50-59------------ 51

60-69------------ 16

70-79------------ 12

Answer: 56.1 mi/h. CAN SOMEONE PLEASE HELP ME WORK THIS OUT?

Solution

Prepare freq distribution using midpoint i.e. 30-39 - 34.5 etc.

Hence average = 56.1