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

Then, 10$ x $ = 4$ y $   or   $ y $ =$ \dfrac{5}{2} x $.

$\therefore$ 15$ x $ + 2$ y $ = 4000

$\Rightarrow$ 15$ x $ + 2$ \times \dfrac{5}{2} x $ = 4000

$\Rightarrow$ 20$ x $ = 4000

$\therefore x $ = 200.

So, $ y $ =$ \left(\dfrac{5}{2} \times 200\right) $= 500.

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

    = Rs. $\left(2400 + 1500\right)$

    = Rs. 3900.