Answer :
20/13 is about eyesight vision, the given measures are explained below
10/20 vision means that one can see at 10 feet what a person with normal vision can see at 20 feet. 15/17 vision means that one can see at 15 feet what a person with normal vision can see at 17 feet. 20/20 vision is considered the standard for normal vision. It is quite common, but not all people have it. 12/20 vision means that one can see at 12 feet what a person with normal vision can see at 20 feet. The best possible human vision is 20/10 vision. This means that one can see at 20 feet what a person with normal vision can see at 10 feet. 10/10 vision means perfect vision. This is very rare and is often referred to as "supervision".
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