Question

Why do fish and frog lay hundreds of eggs where as a hen Lays only one at at time

Answer 1

to increase the chances of egg to get fertilised

Answer 2

Actually female fishes and frogs lay eggs in hundreds and also release sperms in hundreds to fertilize the egg

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This is because there is a chance of egg being eaten by predators and even getting destroyed as they don't have protecting layer.So, laying eggs in hundreds will give a chance for few of them to survive and grow into adult.

But egg of hen has a protective layer hence, it lays only 1 egg