3. The sum of the digits of a two-digit number is equal to some two-digit number, and the digit in the tens place is four times smaller than the digit in the units place. Find this number.
3. First method of solving. Let's list the single-digit numbers for which the second condition is met - one of them is four times smaller than the other. These numbers are: 1 and 4, 2 and 8.

From the obtained pairs, we will choose the one that satisfies the first condition - the sum of the digits should equal a certain two-digit number:
$1+4=5$ - does not satisfy;

$2+8=10-$ satisfies.

Second method of solving. Let's represent the problem condition in the form of a diagram.

![](https://cdn.mathpix.com/cropped/2024_05_21_e53d4ff4f2278a1c0d12g-026.jpg?height=166&width=366&top_left_y=545&top_left_x=822)

Let $x$ be the number of tens. Then $4x$ is the number of units. The smallest two-digit number is 10. We will form an equation:

$$
\begin{aligned}
& x+4x=10 \\
& 5x=10 \\
& x=2
\end{aligned}
$$

Then $2 \times 4=8$.

Therefore, the number 28 satisfies the condition. Similarly, we can form equations for other numbers from 11 to 18 and draw a conclusion.

Third method of solving. Based on the problem condition, the sum of the digits must be divisible by 5. There are two such numbers:

$$
\begin{aligned}
& 10: 5=2 \\
& 2 \times 4=8
\end{aligned}
$$

We get the number 28.

$$
\begin{aligned}
& 15: 5=3 \\
& 3 \times 4=12
\end{aligned}
$$

In this case, we do not get a two-digit number.

Answer: 28.