If the standard deviation of the data .S={5,20,a} is 15. Find the value of a

How to find the standard deviation after we have found the range?

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There are two national college-entrance examinations, the scholastic aptitude test (SAT) and the American College Testing program (ACT).  Scores on the SATs are approximately normal with mean 500 and standard deviation 100.  Scores on the ACTs are approximately normal with mean 18 and standard deviation 6. Use the links provided for the Normal  (Links to an external site.)calculator and the Inverse Normal  (Links to an external site.) calculator to help complete the problems, you do not need to do any work out by hand. (instructions on how to use these calculators can be found in the week 5 star on the home page)  a)  What percent of all SAT scores are above 600? b)  Which is the greater accomplishment, scoring 630 on the SAT or 22 on the ACT ? Explain your reasoning for your choice. c)  How high a score in the SAT is needed to place in the top 2.5%.