The National Bank of Hungary withdrew the 1 and 2 forint coins from circulation on March 1, 2008. In cash purchases in stores, the prices of individual items are not rounded separately, but the final amount is rounded to the nearest five. The following table can be seen in the issued information about the rounding rules:

amounts ending in 1 or 2 are rounded down to 0;

amounts ending in 3 or 4 are rounded up to 5;

amounts ending in 6 or 7 are rounded down to 5;

amounts ending in 8 or 9 are rounded up to 0.

Marci buys two croissants every morning at the local bakery. After a few days, due to rounding, he saves exactly the price of one croissant. We know that the price of a croissant is more than 10 Ft. What could be the unit price of the croissant if this saving occurs as quickly as possible?
Solution. Let's examine the possible cases based on the ending digit of the roll's price from the following table:

| Price  $(\mathrm{Ft} / \mathrm{unit})$ | Total Price | Payable  $(\mathrm{Ft})$ | Savings  $(\mathrm{Ft})$ | Saving Time  $(\mathrm{day})$ |
| :---: | :---: | :---: | :---: | :---: |
| $10 a+0$ | $20 a+0$ | $20 a$ | 0 | - |
| $10 a+1$ | $20 a+2$ | $20 a$ | +2 | $5 a+0.5$ |
| $10 a+2$ | $20 a+4$ | $20 a+5$ | -1 | - |
| $10 a+3$ | $20 a+6$ | $20 a+5$ | +1 | $10 a+3$ |
| $10 a+4$ | $20 a+8$ | $20 a+10$ | -2 | - |
| $10 a+5$ | $20 a+10$ | $20 a+10$ | 0 | - |
| $10 a+6$ | $20 a+12$ | $20 a+10$ | +2 | $5 a+3$ |
| $10 a+7$ | $20 a+14$ | $20 a+15$ | -1 | - |
| $10 a+8$ | $20 a+16$ | $20 a+15$ | +1 | $10 a+8$ |
| $10 a+9$ | $20 a+18$ | $20 a+20$ | -2 | - |

According to the problem, the digit $a \geq 1$ in the roll's price is a positive integer. Under these conditions, the smallest integer value among the expressions in the saving time column is $5 a+3=8$ when $a=1$. That is, the 16 Ft roll can be saved the fastest in 8 days, with a daily savings of 2 Ft.