HELP Write an inequality?!

Question

Write an inequality that represents the numbers that are two standard deviations above and below the mean of the data set: 78, 67, 90, 91, 65, 78, 79.

Answer 1

First calculate the mean or average of these data set which is : (78 + 67 + 90 + 91 + 65 + 78 + 79)/7 = roughly 78

Then find the standard dev:

sqroot{(78-78)^2+(67-78)^2+(90-78)^2+(91-78)^2+(65-78)^2+(78-78)^2+(79-78)^2}/7 = 9.3

Two standards above the mean is 78+(2x9.3) = 97

Two standards below the mean is 78-(2x9.3) = 59

The question, are you using Chebyshev's inequality?