Your equation is 6/25 = d/30. to solve this equation we have to get d by itself! to do this, you have to multiply each side by 30 since d is being divided by thirty. d/30 x 30 = d, 6/25 x 30 = 7.2, or 7 2/10
therefore, d = 7.2
hope this helps!

Since approximately 68% of a (roughly) normal distribution falls within one standard deviation of the mean, that leaves 32% that falls outside, and by symmetry 16% should lie above one standard deviation of the mean.