Question

Are the following in proportion ?

$15,16,20,21$

Answer 1

Given $15,16,20,21$

It written as $15:16::20:21$

First and last term is called extreme terms, that is $15$ and $21$ and $16$ and $20$ is called middle terms.

Product of extreme terms $=$ Product of middle terms

$15\times 21=16\times 20$

Therefore $315\ne 320$

Hence $15,16,20,21$ are not proportion.