Prateek sold a music system to Karthik at 20% gain and karthik sold it to Swastik at 40% gain. If Swastik paid Rs. 10500 for the music system, What amount did Prateek pay for the same ?

Let the price paid by Prateek be Rs. x.
Then,
$$\eqalign{ & 140\% {\text{ of }}120\% {\text{ of }}x = 10500 \cr & \Rightarrow \frac{{140}}{{100}} \times \frac{{120}}{{100}} \times x = 10500 \cr & \Rightarrow x = \left( {10500 \times \frac{{25}}{{42}}} \right) \cr & \,\,\,\,\,\,\,\,\,\,\,\,\, = 6250 \cr} $$