The mean is 18.1 • the z-score for 13.0 is -1.7 what is the standard deviation for this data set?

The standard deviation is 3.

The formula for a z-score is

z = (X-μ)/σ

Using our information, we have

-1.7 = (13-18.1)/σ

Multiplying both sides by σ, 
-1.7σ = 13-18.1
-1.7σ = -5.1

Divide both sides by -1.7:
-1.7σ/-1.7 = -5.1/-1.7
 σ = 3