Question

Assume the random variable x is normally distributed with mean p=85 and standard deviation =4. Find the indicated probability P(x<81)=

Answer 1

Answer:

0.16

Step-by-step explanation:

81 is 1 standard deviation below the mean.  According to the empirical rule, 68% of data lies within 1 standard deviation of the mean; here the equivalent statement would be that half of that, or 34%, lies within 1 standard deviation below the mean.  Half of the area under the standard normal curve, less this 0.34, is the probability that the random variable is below 1 standard deviation beneath the mean.  That comes to 0.16.

On a calculator, you might enter the command normalcdf(-100,81,85,4); on my old TI-83 Plus I get 0.1587, which rounds off to 0.16.