There are actually 3 persons in the riddle and not four.

One is a grandfather, other is the father and the third is the son.

Thus, the son, his father, and his grandfather go for fishing. Thus logically, there are 2 fathers and 2 sons.

Each person catches 1 fish each, so there are a total of 3 fishes only.
2 fathers and 2 sons means 3 persons ------> Let 3 persons be A,B and C... The chain is like this------>A------ B is A's son------ C is B's son...So A and B are 2 father's and B and C are two sons...... ... Now, since there are 3 persons, and each catches 1 fish, so in total there are 3 fishes..
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