Ratio represents the proportion in which a number contains another number. A ratio is represented by a/b or a:b. The duplication or division of every term of a proportion by the same non zero number does not influence the proportion.Subsequently, 3:5 is the same as 6:10 or 9:15 or 12:20 and so on.
The fairness of two proportions is called extent. As 2:3 = 6:9, we compose, 2:3::6:9 and we can say that 2,3,6,9 are in extent . Here 2 and 9 are called extremes while 3 and 6 are called implies.
In an extent, result of extremes=product of means.
if a:b::c:d, then d is known as the fourth relative to a,b and c.
if a:b::c:d, then c is known as the third relative to a and b.
Mean relative is the middle of a and b.