3. Three mryak are more expensive than five bryak by 10 rubles. And six mryak are more expensive than eight bryak by 31 rubles. By how many rubles are seven mryak more expensive than nine bryak
3. Answer: by 38 rubles. Solution. By adding 3 bryaks and myraks, we increase the price difference between myraks and bryaks by 21 rubles. This means that one myrak is 7 rubles more expensive than one bryak. Then seven myraks are more expensive than nine bryaks by $31+7=38$ rubles. Criteria: correct solution - 7 points. The reasoning is correct, but an arithmetic error is made - 5 points. It is established that a myrak is 7 rubles more expensive than a bryak, but the solution is not completed - 2 points.