The monthly incomes of two friends David and Ajay, are in the ratio 5 : 7 respectively and each of them saves ₹78000 every month. If the ratio of their monthly expenditure is 1 : 4, find the monthly income of David(in ₹).

Correct option is B

Given:

Monthly income ratio of David and Ajay: 5 : 7

Monthly savings of both David and Ajay: ₹78000 each

Monthly expenditure ratio of David and Ajay: 1 : 4

Solution:
Let the monthly incomes of David and Ajay be 5x and 7x respectively.
Let their monthly expenditures be y and 4y respectively.

We know that:

Income = Expenditure + Savings

For David:

5x = y + 78000

For Ajay:

7x = 4y + 78000

From the first equation, for David:

y = 5x - 78000

Substitute this value of y into the second equation for Ajay:

7x = 4(5x - 78000) + 78000

7x = 20x - 312000 + 78000

7x = 20x - 234000

−13x = −234000

x = $$\frac{234000}{13}$$​

x = 18000

Income of David = 5x = 5$$\times$$​ 18000 = 90000

The monthly income of David is ₹90,000.