A reduction of 21% in the price of wheat enables a person to buy 10.5 kg more for Rs. 100. What is the reduced price per kg ?

Let the original price = Rs. $$x$$ per kg
Reduced price = Rs. $$\left( {\frac{{79x}}{{100}}} \right)$$ per kg
$$\eqalign{ & \therefore \frac{{100}}{{79x}} - \frac{{100}}{x} = 10.5 \cr & \Rightarrow \frac{{10000}}{{79x}} - \frac{{100}}{x} = 10.5 \cr & \Rightarrow 10000 - 7900 = 10.5 \times 79x \cr & \Rightarrow x = \frac{{2100}}{{10.5 \times 79}} \cr} $$
∴ Reduced price :
$$\eqalign{ & = {\text{Rs}}{\text{. }}\left( {\frac{{79}}{{100}} \times \frac{{2100}}{{10.5 \times 79}}} \right){\text{ per kg}} \cr & = {\text{Rs}}{\text{. 2 per kg}} \cr} $$