In 20042005 the mean annual cost to attend a private univers

In 2004-2005 the mean annual cost to attend a private university in the United States was $21158.

Solution

P(X>x)=0.9

--> P((X-mean)/s <(x-21158)/4500)=1-0.9=0.1

--> P(Z<(x-21158)/4500) = 0.1

--> (x-21158)/4500= -1.28 (from standard normal table)

So x= 21158 -1.28*4500 =15398