1. **State the problem:** We are given that 6 kg of biscuits cost 27. We need to find the cost of 13 kg of biscuits. 2. **Identify the relationship:** The cost is directly proportional to the weight since the price per kg is constant. 3. **Find the cost per kg:** $$\text{cost per kg} = \frac{27}{6} = 4.5$$ 4. **Calculate the cost for 13 kg:** $$\text{cost for 13 kg} = 13 \times 4.5 = 58.5$$ 5. **Conclusion:** The cost of 13 kg of biscuits is 58.5.