Question

Suppose a normal distribution has a mean of 79 and a standard deviation of 7. What is P(x ≥ 72)?

Answer 1

Approximately 65% of the distribution lies within one standard deviation of the mean, which is to say,

P(72 ≤ x ≤ 86) ≈ 0.65

Normal distributions are symmetric, so the percentage of values one standard deviation below the mean is equal to the percentage of values one standard deviation above the mean.

P(72 ≤ x ≤ 79) = P(79 ≤ x ≤ 86)

but since the sum of these make up P(72 ≤ x ≤ 86), we find

P(72 ≤ x ≤ 79) ≈ 0.65/2 = 0.325

Also due to symmetry, exactly half of the distribution lies to either side of the mean; namely,

P(x ≥ 79) = 0.5

It follows that

P(x ≥ 72) = P(72 ≤ x ≤ 79) + P(79 ≤ x)

P(x ≥ 72) = 0.325 + 0.5

P(x ≥ 72) = 0.825 ≈ 0.84