6. The number of trees planted by students during the clean-up day is a three-digit number, in which the number of tens is 3 more than the number of hundreds, and the number of units is 4 less than the number of tens, and the half-sum of all digits is equal to the digit of the tens. How many trees did the students plant?
6. Let tens be $x$, then hundreds are $x-3$, and units are $x-4$. According to the problem,

$$
\frac{x+(x-3)+(x-4)}{2}=x
$$

from which $x=7$. Therefore, the desired number is 473.