Problem 5.5. On some trees in the magical forest, coins grow. The number of trees that do not grow any coins at all is twice as many as the trees that grow three coins. On three trees, two coins grow, on four trees - four coins, and no tree grows more than four coins. By how much is the total number of coins in the magical forest greater than the number of trees?
Answer: 15.

Solution. Let $x$ be the number of trees in the forest on which three coins grow, and on which one coin grows. Then in the forest, $2 x$ trees do not grow any coins at all.

Thus, the total number of coins is

$$
2 x \cdot 0+y \cdot 1+3 \cdot 2+x \cdot 3+4 \cdot 4=3 x+y+22
$$

and the total number of trees is

$$
2 x+y+3+x+4=3 x+y+7
$$

The first number is 15 more than the second.