If X is a normal random variable with mean 11, and if the probability that X is less than 12.10 is .72 , then what is the standard deviation of X?

Question

Select one:
a. 3.00
b. 6.25
c. 2.50
d. 1.25

Please note that the answer for the above is stated as c.2.50. However, my answer comes to 1.89 which is not given as an option and I can't figure out where I'm going wrong so please can anyone help me with this problem.

Thanks.

Answer 1

You are correct. I also get 1.89 for the standard deviation.

If the standard deviation were 2.50, only 67% of X numbers would be below 12.10

Answer 2

Thank you!