A look at a graph of the sampling distribution of M

The blue region of the graph is 15 or higher and is a small portion of the area. The probability can be determined by computing the number of standard deviations above the mean 15 is. Since the mean is 10 and the standard deviation is 2.35, 15 is (15-10)/2.35 = 2.13 standard deviations above the mean. From a z table it can be determined that 0.983 of the area is below 2.13; therefore, 0.017 of the area is above. It follows that the probability of a difference between the drug and no-drug means of 15 or larger is .017.

As shown below, the z table calculator allows you to compute this value directly without using z scores.