Let the initial count of his fish be n.
He gives away 2/3 of these fish: 2n/3, leaving n/3 fish.
He gives away 2/5 of these remaining n/3 fish: (2/5) times (n/3), or
2n
--- , meaning that (3/5) times (n/3) fish are left.
15
(3/5)(n/3) = n/5 fish left = 30 fish. Thus, n = 150 fish.
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