At the place of sale a sign:
1 strawberry - 13 hryvnia
3 strawberries - 40 hryvnia
The buyer:
Give me one strawberries.
Pay 13 hryvnia, take the strawberries. This is repeated twice again.
The buyer:
"I bought three watermelons from you for 39 hryvnia, although it is written that you sell them for 40. How stupid you are!
The Merchant:
- And so all the time... They take three watermelons and teach me how to trade properly!