João lives in Salvador and his parents in Recife. To alleviate his longing, he calls his parents every three days. The first call was made on a Sunday, the second call on the following Wednesday, the third call on Saturday, and so on. On which day of the week did João call his parents for the hundredth time?
As João calls his parents every three days, we can set up a table indicating the days of the week on which João's first fourteen calls occurred.

Sunday Monday Tuesday Wednesday Thursday Friday Saturday

| 10 | $6^{\circ}$ | $4^{\circ}$ | $2^{\circ}$ | $7^{\circ}$ | $5^{\circ}$ | $3^{\circ}$ |
| :---: | :---: | :---: | :---: | :---: | :---: | :---: |
| $8^{\circ}$ | $13^{\circ}$ | $11-$ | 9 | $14-$ | $12^{\circ}$ | $10^{\circ}$ |

Analyzing the first row of this table, we notice that there are seven calls, one on each day of the week, and that starting from the seventh call, the days begin to repeat. This implies that the numbers that appear in the second row of the table are obtained from the numbers that appear in the first row by adding 7. For example, João will call his parents on Sundays for calls numbered

$$
\begin{array}{r}
1 \\
1+7=8 \\
8+7=15 \\
15+7=22 \\
22+7=29 \\
29+7=36
\end{array}
$$

that is, in the numbers that leave a remainder of 1 when divided by 7. With this reasoning, we can determine the day of the week on which a call falls by analyzing the remainder of the division of the call number by 7.

| Sunday | Monday | Tuesday | Wednesday | Thursday | Friday | Saturday |
| :---: | :---: | :---: | :---: | :---: | :---: | :---: |
| 1 | 6 | 4 | 2 | 7 | 5 | 3 |
| 8 | 13 | 11 | 9 | 14 | 12 | 10 |
| $\vdots$ | $\vdots$ | $\vdots$ | $\vdots$ | $\vdots$ | $\vdots$ | $\vdots$ |
| $\uparrow$ | $\uparrow$ | $\uparrow$ | $\uparrow$ | $\uparrow$ | $\uparrow$ | $\uparrow$ |
| remainder 1 | remainder 6 | remainder 4 | remainder 2 | remainder 0 | remainder 5 | remainder 3 |

Dividing 100 by 7, we get $100=7 \times 14+2$. Therefore, the remainder of the division of 100 by 7 is 2, and it follows that the hundredth call occurs on a Wednesday.