Estimating a pond population by tag and recapture

You catch $50$ fish from a pond, tag them and let them go. Next day you catch $40$ fish and find $10$ carrying tags. What is the natural estimate of how many fish live in the pond?

$200$. A quarter of the second sample is tagged, so the $50$ tagged fish should be about a quarter of the population (Lincoln–Petersen estimate $\tfrac{50\times40}{10}$).