Jack and his father and, John and his father went fishing. At the end of the day, the fathers had caught twice as many fish as had the sons. Further, Jackís father had caught twice as many fish as had Johnís father.
If 35 fish were caught in all, and each person caught a whole number of fish - who is older, Jack or John?
Well, if we are dealing with 4 distinct people, then the number of fish caught would need to be x + y + 2x + 2y, which is divisible by 3. But 35 is not divisible by 3, so there are only 3 people here, a son, a father, and the father's father. Since Jack's father caught twice as many fish as John's father, Jack is father to John, and necessarily the older of the two (unless there is a strange adoption scenario).
The problem doesn't require calculation of the actual catch, but it is not hard. x + 2x + 4x = 35, so x = 5. The fish caught were:
Jack (John's father): 10
Jack's father (John's grandfather): 20