Question

The average cost of an irs form 1040 tax filing at thetis tax service is $150.00. assuming a normal distribution, if 90 percent of the filings cost less than $172.00, what is the standard deviation? hint: use excel or appendix c-2 to find the z-score. (

Answer 1

P(Z<z)=0.9 => z=1.281552 from normal probability tables.

z=(x-mean)/standard deviation =>
standard deviation =(x-mean)/z=(172-150)/1.281552=17.17