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 Sunday, the second call on Wednesday, the third call on Saturday, and so on. On which day of the week did João call his parents for the hundredth time?
Given that João calls his parents every 3 days, we can create a table that indicates the days of the week on which João's first 14 calls occurred:

| Sunday | Monday | Tuesday | Wednesday | Thursday | Friday | Saturday |
| :---: | :---: | :---: | :---: | :---: | :---: | :---: |
| 1 | 6 | 4 | 2 | 7 | 5 | 3 |

Analyzing the first row of this table, we notice that there are 7 calls, one on each day of the week, and that starting from the 70th call, the days begin to repeat. This implies that the numbers in the second row of the table are obtained by adding 7 to the numbers in the first row.

For example, João will call his parents on Sundays for the calls numbered:

\[
\begin{aligned}
& 1 \\
& 1+7=8 \\
& 8+7=15 \\
& 15+7=22 \\
& 22+7=29 \\
& 29+7=36
\end{aligned}
\]

that is, on the numbers that leave a remainder of 1 when divided by 7.

With this reasoning, we can determine the day of the week for a call by analyzing the remainder of the call number divided 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 100 divided by 7 is 2, and it follows that the 100th call will be on a Wednesday.