The breaking strengths (say, y) for one foot square samples of a particular synthetic fabric are approximately normally distributed with a mean of 2280 pounds per square inch and a standard deviation of 10.6 psi.

Question

a) find the probability of selecting a 1 foot square sample of material at random that on testing would have a breaking strength in excess of 2240 psi.

b)what is the breaking strength value in psi that separates the strongest 20% of one foot square samples from the weakest 80%?

Answer 1

a) Z = (score-mean)/SD

Find table in the back of your statistics text labeled something like "areas under normal distribution" to find the proportion/probability related to the Z score.

b) Start with Z value for .20 and insert i8n equation above.