Explanation:
For example:
Ex: 2:3 :: 6:9
In this question, the means are 3 and 6.
The extremes are 2 and 9.
We use this method to find whether the two ratios are equal or not.
We multiply the means and the extremes, and if there products are same, then these two ratios are proportional.
If these two ratios are proportional, their "means" = "extremes"
=> Means = Extremes
=> 3 x 6 = 2 x 9
=> 18 = 18
So, these two ratios are proportional.
Hope this helps you.