Let x= cost of fifth book Sum of numbers (cost of books) = Cost of the four books + cost of the fifth book Sum of cost of books =11.75+x Number of numbers (cost of books) =5 But the mean of the cost of books =3.25 Get rid of the fraction by multiplying both sides of the equations by 5 11.75+x=16.25 x=16.25−11.75 x=4.50