Math ?

sketch a normal curve for each distribution. label the x-axis values at one, two, and three standard deviations from the mean.

15. mean = 30, standard deviation = 4

You share only one distribution, so we'll focus on that one:  mean:  30; std. dev.:  4.  

Draw a "standard normal curve."  Draw a vertical line in the exact middle of your curve.  Label this line "30."  Now "one standard dev. above the mean" is 30+4=34; "two std. devs. above the mean is 30+4+4=38, or 30+8=38.  "three std. devs. above the mean is 30+3(4) = 42.  

Now work in the other direction.  Start with the mean:  30.  But now subtract the std. dev. (4) instead of adding it.  You'll get 30-4=26.  This is "1 std. dev. below the mean.  Continue:  find 2 and 3 std. devs. below the mean.