The mean of a population is 1200 80 of the date is within 15

The mean of a population is 1200, 80% of the date is within 1.5 standard deviations of mean and lies between 900 and 1500, What is the standard devication?

- The book said that I should use z score equation and I don\'t understand. Can you explan detail, pelase~

Solution

1500-900 = 600

Since this length 600 is same as 3 * std dev (given), std dev = 600/3 = 200