**Quote:**

A fisherman caught 4000 fish. He marked at least 1/4 of the fish with a single mark. He also marked 400 fishes with two or more marks. What is the probability that the fisherman will catch a fish with a single mark among all the marked fishes?

Did you actually mean "at least" in the question. If that is the case, i dont think u can solve the problem,

Assuming the question meant exactly 1/4.

then Fishes with one mark = 1000.

fishes with 2 or more = 400.

so reqd probability = 1000/(1000 + 400) = 10/14 = 5/7.

If the question actually meant atleast 1/4. then it can be any number greater than 1000. so if the number of single marked fishes is 1/2 the answer wud be 2000/24000 = 5/6.

you can have effectively 1000 to 3600 as the number of single marked fishes. and each of these will give you a different probability.