Evdokimov M.A.

In the Country of Fools, coins of 1, 2, 3, ..., 19, 20 soldo (no others) are in circulation. Buratino had one coin. He bought an ice cream and received one coin as change. He bought the same ice cream again and received change in three different coins. Buratino wanted to buy a third such ice cream, but he did not have enough money. How much does the ice cream cost?
Paying with three different coins amounts to no less than $1+2+3=6$ soldos. Since this amount was not enough to buy the ice cream, it must cost no less than 7 soldos.

The ice cream cannot cost more than 7 soldos; otherwise, two ice creams would cost no less than $8+8=$ 16 soldos. However, Buratino had only one coin, which is no more than 20 soldos, and he would not have been able to get 6 soldos in change. Therefore, the ice cream can only cost 7 soldos.

The payment process for the ice cream looked like this: $20-7=13, 13-7=6=1+2+3$.

## Answer

7 soldos.