Quiz

     Mr. Sawada wanted a pen, so he went to a stationary store to buy the pen.
The stationary store had two kinds of pens. One is 100 yen, and the other is 80 yen.
He wanted 100 yen's one. He gave the shopkeeper 100 yen. Then the shopkeeper gave Mr. Sawada 100 yen's pen with a silent. Mr. Sawada  went back home with a joy. However, why the shopkeeper gave Mr. Sawada 100 yen's pen? Perhaps Mr. Sawada might want 80 yen's pen and change, 20 yen.