75. When Vanya was asked how old he was, he thought for a moment and said: “I am three times younger than Dad, but three times older than Seryozha.” At this point, little Seryozha ran up and reported that Dad is 40 years older than him. How old is Vanya?
75. Dad is 3 times older than Vanya, who, in turn, is 3 times older than Seryozha, so Dad is 9 times older than Seryozha. Therefore, Dad is 8 times older than Seryozha, which is 40 years. Hence, Seryozha's age is 5 years. Vanya is three times older than Seryozha, so he is 15 years old.

109 Such problems!