4. On the car market, you can exchange three Ladas for one Volga and one Mercedes, and three Volgas for two Ladas and one Mercedes. Will collector Vasya, having 700 Ladas, be able to get 400 Mercedes?
Answer: No. Solution. Let's estimate the Zhiguli at 4 rubles, the Volga at 5 rubles, and the Mercedes at 7 rubles. Then, the total value of Vasya's cars will be preserved with any exchanges. The total value of 400 Mercedes is exactly equal to the total value of 700 Zhigulis, but it is impossible to exchange Zhigulis for only Mercedes, because with such exchange rules, a Mercedes will always come with at least one other car.

Note. The "prices" of the cars are any solution to the system of equations $3 Ж=B+M, 3 B=2 \mathcal{N}+M$.

Grading guidelines. The concept of cost is introduced, the system of equations is correctly formulated, it is shown that $7 Ж=4 M$, but there is no further substantial progress -3 points. The concept of cost is introduced, at least one set of costs that is preserved during exchanges is found, but there is no further substantial progress -3 points. The concept of cost is introduced, the system of equations is correctly formulated, but there is no further substantial progress -2 points. The answer "yes" with justification involving adding a foreign car and then giving it back, without analyzing the problem in the correct interpretation -1 point.