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the systolic blood pressures have a normal distribution with a average 120 and a sd 8. the percentage of individuals with a systolic blood pressure above 120 is

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2 Answers

Hi Ruth Ann --

A handy concept in stats: half the group is above average or mean value, and half would be below.

An "average" generally cuts the group in halves.

You need to convert this problem to a standard normal distribution to calculate the probability.  That means you convert the systolic number 120 to a Z-score.  Z = ( X - μ ) / σ, and you know all three variables.

Z = (120 - 120)/8 = 0/8 = 0.

P(X > 120) = P(Z > 0) = 0.5000, which can easily be looked up on a standard normal distribution probability table, or hopefully you remember that the normal curve is symmetric around Z=0 such that half of the area under the curve falls on either side of Z=0.