I'm having a hard time trying to figure out how to start off on this problem. so the question asks:

"The Bureau of Land Management did a study of the water table near Custer, Wyoming, in the month of June. Based on data from the Bureau, a random sample of 20 water wells showed that the distances from the ground to the water level (in feet) were

Distance from the ground to water level:12-14 | 15-17 | 18-20 | 21-23 | 24-26

Number of Wells f: 1 | 3 | 8 | 2 | 6

would I start off by finding the mean of each value from "Distance from the ground to water level"? Such as (12+13+14)/3=13 would be the mean for 12-14?

then 1(13)+ the rest?

or am i supposed to do something else

The question asks to find:

a. Sample Mean

b. Sample Variance

c. Sample Standard Deviation

EDIT:I think the 12-14, etc are categories, which the other numbers are frequencies. If that's the case, I may know how to solve this. It'd be great if anybody can confirm this.