# Probability Blues

• Jul 18th 2010, 03:46 AM
cyt91
Probability Blues
A sales contract between a manufacturer and a buyer requires 20 towels to be subjected to a water absorption test. If no more than 1 towel fails the test, the batch is accepted. If 2 or 3 towels fail the test, an additional 20 towels are tested. The batch is then accepted if 3 or less (out of 40) fail the test. Otherwise, the batch is rejected.

(a) If a batch of towels contains 5 % which would be rejected by the test,what is the probability that the batch is accepted.

(b) The lengths of 20 towels are measured and if the mean length is less than a value a specified in the contract, the batch is rejected. What should the value of a be to give a probability of 0.99 of accepting a batch with mean length of 106 mm and standard deviation of 6 mm.

I solved (a). The probability that the batch is accepted is 0.8961.

But I can't solve (b).
• Jul 18th 2010, 04:10 AM
mr fantastic
Quote:

Originally Posted by cyt91
[snip]

(b) The lengths of 20 towels are measured and if the mean length is less than a value a specified in the contract, the batch is rejected. What should the value of a be to give a probability of 0.99 of accepting a batch with mean length of 106 mm and standard deviation of 6 mm.

[snip]

You need to find the value of a such that $\Pr(X \geq a) = p$ where X ~ Normal $(\mu = 106, \, \sigma = 6)$ and $p^{20} = 0.99 \Rightarrow p = ....$

So it's more or less a routine inverse normal problem and your class notes and textbook will have examples to review.

If you need more help, please show what you've tried and say where you get stuck.