Richard H. answered • 06/13/19
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The first step is to convert the high and low scores to z scores by dividing the difference between the mean and the score of interest by the standard deviation. Here you get +0.50 and -0.50. Now look at your standard deviation table, specifically the top, where it likely has a picture showing what is the area under the curve. Many z-tables have it set up that there are only positive z scores, but that is okay because measuring to the middle (z =0) a negative z has the same area as a positive z. Here you would look up z=0.50 and double the answer because of the symmetry. To get a percentage, multiply the sum by 100.
