Find the greatest number that will divide 63, 45 and 69 so as to leave the same remainder.

According to the empirical rule, roughly 68% of the normal distribution is within 1 standard deviation of the mean.

In this case, that tells us 68% of the population is between x = 65 and x = 79. The center is at mu = 72, which is the highest point. On either side of this center is where the curve goes downward as you move away from the center. Note the symmetry as well. Each tickmark represents 1 standard deviation unit.

Because 68% of the class got scores between x = 65 and x = 79, this means 100% - 68% = 32% of the class got a score x such that x < 65, or x > 79

Cut this in half and get (32%)/2 = 16%

16% of the population is in either tail.

16% of the population is the set of x values such that x < 65; we can say that 16% of the population is more than 1 standard deviation from the mean, and also smaller than the mean.

The 16th percentile is the marker where 16% of the population is below that value.

Therefore, the 16th percentile is the value  65  

16% of the class scored less than 65 on the test.