The following data represent the length of time (in minutes) between eruptions of Old Faithful in Yellowstone

Question:

The following data represent the length of time (in minutes) between eruptions of Old Faithful in Yellowstone National Park.
Time (minutes) .... Frequency
40-49 ........ 8
50-59 ........ 44
60-69 ......... 23
70-79 .......... 6
80-89 ......... 107
90-99 ......... 11
100-109 ........ 1
(a) Approximate the mean length of time between eruptions.
(b) Approximate the standard deviation length of time between eruptions.