variability

sdeutz31

1. The problem statement, all variables and given/known data

# of pebble at this size millimeter

1 71mm
3 77mm
2 78mm
4 79
5 80
12 81
19 82
14 83
17 84
23 85
20 86
23 87
21 88
15 89
9 90
8 91
1 92
1 93

note: the avg. size is 84.46

Suppose d is the avg. pebble size, then the variability is * such that 68% (somewhat more two thirds) of the sizes fall between d-* and d+*. Estimate by eye the cariability in the sidtribution shown.

How do you go about finding this...im so confused