1. **State the problem:** We are given the cost of 12 quintals of soyabean as 36000 rupees and need to find the cost of 8 quintals. 2. **Formula used:** Cost is directly proportional to quantity when the price per unit is constant. So, we use the formula: $$\text{Cost}_1 : \text{Quantity}_1 = \text{Cost}_2 : \text{Quantity}_2$$ 3. **Apply the values:** $$36000 : 12 = x : 8$$ 4. **Solve for $x$ (cost of 8 quintals):** $$x = \frac{36000 \times 8}{12}$$ 5. **Calculate:** $$x = \frac{288000}{12} = 24000$$ 6. **Answer:** The cost of 8 quintals of soyabean is $24000$.