For a recent history test, scores follow the normal distribution with a mean of 70 points. 80% of the students scoblack below 88 points. What is the standard deviation of the scores?

I have done a lot of research into the question and eventually broke down and used the standard table. The answer I got through that was a std of 21.387.

I know I need to use the z score formula (x-mean)/std = z but I am searching for two of those variables and would need the z score at least to find the std.

A push in the right direction would be greatly appreciated.

I have done a lot of research into the question and eventually broke down and used the standard table. The answer I got through that was a std of 21.387.

I know I need to use the z score formula (x-mean)/std = z but I am searching for two of those variables and would need the z score at least to find the std.

A push in the right direction would be greatly appreciated.