Dear Experts

A customer goes to a shop and purchase five products (from A to E)

The shopkeeper makes bill of =1815
Now customer asks for discount of = 615
He want to pay only 1200 instead of =1815

How to adjust rates so that the bill amount will be 1200 with out any changes in quantity
Product and Rate table is attached.

Item-Qty--Rate--Amount
A----13----60----780
B----8-----40----320
C----7-----35----245
D----18----15----270
E----25----8-----200

Bill Amount= 1815

Please help