Answer with Explanation:

Let the cost of a chair and that of a table be Rs. x and Rs. y respectively.

Then, 10x = 4y   or   y = 5/2x.

 15x + 2y = 4000

 15x + 2 x 5/2 x = 4000

 20x = 4000

 x = 200.

So, y = (5/2x 200) = 500.

Hence, the cost of 12 chairs and 3 tables = 12x + 3y

    = Rs. (2400 + 1500)

    = Rs. 3900.