Question

A fat person can swim easier then thin person because of which rule

Answer 1

because the density of water a fat person can swim easier than a thin person

Answer 2

helllo frnds.
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Fat is made of saturated oils. Oil floats on the water and hence fat becomes dry. It should give more generosity.
On the other hand, the muscle is 15% density of adipose tissue. And adipose tissue is 8% less dense than water. (Thanks to Kim Haroon for correcting me with the right facts: What will be the answer of Kim Aaron? A slim person easily floats on water compared to a thin person?)
So a fat swimmer can look a bit easier than the thin one (all the other things are equal). So a thin and muscular person would be really dense and less encouraging than a dense-looking fat person.
Fat cover (under the skin) helps in adopting body from heat loss. So if you are swimming in cold water / weather, then it should help to some extent in reducing the loss of heat.
Organisms living in cool water like whales, seals collect energy in fat and also save energy by preventing heat loss. But they have evolved for very efficient use of energy and their diet also contains high-fat meat. If a person lives in a lifestyle after a high-fat diet for humans then this can not be a good idea.
It can not help better with swimming but swimming can definitely help in losing fat.

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THANKS YOU

@SRK6