3B. Bojan tells Pavel: "I am 2 times older than you were when I was as old as you are now. When you are as old as I am now, the sum of our ages will be 63." How old is each of them?
Solution. If we denote Bojan's age by $x$ and Pavel's age by $y$, then from the conditions of the problem, we form the following system of linear equations:

$$
\left\{\begin{array} { l } 
{ x = 2 ( y - ( x - y ) ) } \\
{ x + ( x + ( x - y ) ) = 6 3 }
\end{array} \text { or } \left\{\begin{array}{l}
3 x-4 y=0 \\
3 x-y=63
\end{array} .\right.\right.
$$

If we solve this system, we find that Bojan is 28 years old, and Pavel is 21 years old.