Problem - Total monthly income of two persons is Rs.7500. They spend 90% and 80% of their incomes?

"Total monthly income of two persons is Rs.7500." I like persons to have names. Let's call them Alice and Bob and define:

a := Alice's monthly income, and
b := Bob's monthly income.

Now we can express the first sentence by the equation:

1) a + b = 7500.

Next we are told that they spend 90% and 80% of their incomes respectively. The ratio of their monthly savings is 3:4.

Alice spends 90%, so she saves 10%. That's 0.1a monthly savings.
Bob spends 80%, so he saves 20%. That's 0.2b monthly savings.

Now we can express this information by the equation:

0.1a / 0.2b = 3/4 ;; the next couple steps make this equation nice and handy
a / 2b = 0.75
a = 1.5b

and we arrive at the second equation [eq.], to complete the system of two linear equations:

2) a - 1.5b = 0

Subtract eq. 2) from eq. 1) and you get:

3) a - a + b - (-1.5b) = 7500 - 0

2.5b = 7500
b = 3000

Insert b into eq. 1) to determine a :

a + 3000 = 7500
a = 4500

So, finally, we know that

Alice's monthly income is Rs. 4500, and
Bob's monthly income is Rs. 3000.