1. ## Distribution

I need help with a problem which I can not solve. I hope you can help me.

Question:
The time needed to complete a final examination in statistics course is normally
distributed with a mean of 80 minutes and a standard deviation of 10 minutes.

a) What is the probability of completing the exam in one hour or less?

b) What is the probability that a student will complete the exam in more than 60
minutes but less than 75 minutes?

c) Assume that the class has 60 students and that the examination period is 90
minutes in length. Find the probability that 60 randomly selected students
will have a mean time to complete the exam greater than 90 minutes.

2. ## Re: Distribution

If someone can explain the answer for the first question only it would be perfect.

3. ## Re: Distribution

$\displaystyle P(X<60)=P\left({X-\mu\over\sigma}<{60-80\over10\right)=P(Z<-2)\approx .0228$

4. ## Re: Distribution

Ok, I understand the formula. But how do u come on the answer: 0.0228

5. ## Re: Distribution

I am sure your book has a st normal table and that your professor covered that.