How Many Fish are in all the World’s Seas?

What they want to understand is what is your approach to that brainteaser. In this case i would suggest in a general perspective to do the following:

Start saying that in each KM3 there are x Number of Fish, then you can proceed by calculating the numer of fish in each ocean, having in mind that there are a lot of differences in the composition of the Atlantic´s ocean than the Dead sea.

Based on that, you should calculate the volume of each ocean and then making multiplying it by the number of fish in each of them. if you want to go further in your analysys, you could come up with the fish born seasonality, which could impress your interviewer but at the same time could mean much difficult calculations.

Hope it works.

Best,

Juan Pablo Montoya

I would drastically simplify how to calculate the size of the sea by starting with assumption that there is a maximum sea depth that is compatible with most life, call it 30, 50, 100 meters. Once you agree on that you can approximate all seas to a composition of cuboids vs. spheres, making calculations faster and smoother. Once calculated the volume, apply assumed population per cubic kilometer and voila, done.

hope it helps,

andrea