- Assume that Z is a normal random variable with mean 0 and variance 1.

P(Z≥-1)=? P(Z≤?)=0.20
If someone can show either one of these I will be able to work the other I am confident.

2. A random variable has a normal distribution with μ=62.4. Find its standard deviation if the probability is 0.20 that it will take on a value greater than 79.2.
everytime I work this one out I get a negative SD. Basically hat i did was take the Z score from the table forthe 20% which was -.96. Then did the z= x-u/SD and re arranged to find it. 79.2-62.4/-.96 is a negative SD and not possible.