A company that manufacturers bookcases finds that the average time it takes an employee to build a bookcase is 23 hours with a standard deviation of 8 hours. A random sample of 64 employees is taken. What is the likelihood that the sample mean will be 18 hours or more?
Use z-scores:
z = (x - mean)/(sd/√n)
x = 18
mean = 23
sd = 8
n = 64
Calculate z, then use a z-table to determine probability.
I hope this will help get you started.