Suppose A2 is the name of the book B2 is the notional price of the book and C2 is the no. of books purchased, D2 is the discounted price (row 1 is for column headins)
then type this formula in D2
=if(c2>50,B2*0.65,B2*0.80)
you can copy D2 down for other books. easy ways of copying D2 is to take the curor to the right bottom of D2 and it turns into + and click this +.(provided there is not gap row)