Two types of tea costing Rs. 180/kg and Rs. 280/kg. In what ratio should these be mixed so that obtained mixture sold at Rs. 320/kg to earn a profit of 20% is ?

According to the question,
Selling price of the mixture = Rs. 320, Gain = 20%
∴ Cost price of the mixture
$$\eqalign{ & = 320 \times \frac{{100}}{{120}} \cr & = {\text{Rs}}{\text{. }}\frac{{800}}{3} \cr} $$
Now using alligation method

$$\eqalign{ & {\text{Ratio of quantity }} \cr & \to 40:260 \cr & \,\,\,\,\,\,\,\,\,\,\,2:13 \cr} $$