21. If $x, y$ and $z$ are positive integers such that $27 x+28 y+29 z=363$, find the value of $10(x+y+z)$.
21. Answer: 130
Since $x, y$ and $z$ are positive, we have
$$
27(x+y+z)12.5 \text {. }
$$

Since $x, y$ and $z$ are integers, we obtain $x+y+z=13$. Hence $10(x+y+z)=130$.