A man sold two chairs at Rs. 1200 each. On one he gained 20% and on the other he loss 20%. His gain or loss in the whole transaction is:

Answer: Option C

In the case where loss and gain percentage is common on same selling price, always a loss incurs in total deal. And this can be calculated by a short-cut:
Loss on total deal,
$$\begin{array}{rl}& ={\left(\frac{\text{Common loss or gain percentage}}{10}\right)}^2\\ & ={\left(\frac{20}{10}\right)}^2\\ & =4\%\end{array}$$

Alternatively, It can be also calculated through Graphic Change Method: This can be given by,
100 == 20% gain ⇒ 120 == 20% loss ⇒ 96
Loss = 4% (As 100 became 96)