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In this problem x stands for the mean and SD the standard deviation. With the mean = 72 and SD = 4, the students with pulses between 68 and 76 are all of the students within one standard deviation of the mean. In general, about 68% of the population falls within one standard deviation of the mean, 95% within two standard deviations and 99.7% within three standard deviations.
To answer the question, 68% of 200 is .68 * 200 = 136. So 136 students have pulses in the range of 68 and 76 beats per minute.
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