7. Bruno set a four-digit even number as the unlock code for his mobile phone. To avoid forgetting the number, he wrote down the following:

- All digits are different, and the sum of all digits is 15.
- The unit digit is three times smaller than the thousands digit.
- The tens digit is smaller than the unit digit.

Determine the code that Bruno set for unlocking his mobile phone.
First method:

If $a, b, c$ and $d$ are respectively the thousands, hundreds, tens, and units digits of the four-digit number $\overline{a b c d}$, we have:

$$
a+b+c+d=15
$$

Since the desired number must be even, the units digit can be $0, 2, 4, 6$, or 8. The units digit is three times smaller than the thousands digit, which means the thousands digit is three times larger than the units digit.

If $d=0$, then $a=3 \cdot 0=0$, which is impossible because the number is four-digit. 1 POINT If $d=2$, then $a=3 \cdot 2=6$.

If $d=4$, then $a=12$, which is impossible because $a$ is a digit, and the same applies for $d=6$ or 8.

Since $d=2$ and $a=6$, and the tens digit is less than the units digit, i.e., $c<d$, we conclude that $c=0$ or $c=1$.

Now we have two cases:

- If $c=0$, then $b=15-6-0-2=7$, so the desired number is 6702.
- If $c=1$, then $b=15-6-1-2=6$, which is impossible because the digits of the number must be different.