The monthly incomes of two families A and B are shown by the following diagrams:

If the income of the family B is Rs. 12000, then the income of family A would be:

Ratio of the monthly incomes of families A and B
= Ratio of the areas of rectangles A and B
= 5 × 2 : 4 × 1
= 10 : 4
= 5 : 2
Let the monthly income of family A be Rs. x
Then,
5 : 2 :: x : 12000
⇒ 2x = 5 × 12000 = 60000
$$\eqalign{ & \Rightarrow x{\text{ = }}\frac{{60000}}{2} \cr & = 30000 \cr} $$