Question

The drying times for a certain type of cement are normally distributed with a standard deviation of 30 minutes. A researcher wishes to estimate the mean drying time for this type of cement. Find the sample size needed to assure with 68.26% confidence that the sample mean will not differ from the population mean by more than 5 minutes.
a. 31
b. 26
c. 29
d. 38
e. 30
f. 27
g. 32
h. 36

Answer 1

Answer:

h. 36

Step-by-step explanation:

For a 68.26% confidence, z = 1.0.

Standard deviation (s) = 30 minutes

The sample size needed to assure that the sample mean does not differ from the true mean by more than 5 minutes is given by:

$5\geq z*\frac{s}{\sqrt n}\\\sqrt n\geq 1*\frac{30}{5}\\n\geq 36$

The sample size needed is 36.