In species who have a small chance of survival of offspring there are usually a large amount of eggs produced. There is a correlation between the survival rate of offspring and the amounts of eggs produced.
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In frogs fertilization takes place outside the body(external fertilization) so a large number of eggs are released by them to improve the chance of fertilization. Also they can be destroyed by predators as they do not have any protection. So the large no enables adequate number of eggs for fertilization even if some eggs are destroyed.
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