[Yoda]

KOC's friend is the owner of a golf shop. Each box of Pro V is at cost of $250. And after a profit of $70 added onto it, each box of Pro V is sold at $320. So one day, a customer came in to buy a box, and handed the owner a $500 dollar bill. The owner just went out of change, and had to go over to his neighbor to change the 500 dollar bill into bills of different denominations. So after giving the customer the change, and the customer left, the neighbor came and claimed that the $500 dollar bill was fake. So the owner had to give the neighbor another $500 dollar bill to settle the dispute.

The question is: how much money KOC's friend lost at the end of the day?

Anybody who would accept a $500 bill without verifying its authenticity has only himself to blame. Those bills haven't been produced by the U.S. Treasury since World War II and haven't been distributed since 1969 (that would be forty years). While they are still legal tender, ask yourself: "When was the last time I saw a $500 bill? Would I accept it as payment? Would I accept it and make change?"