When three different varieties of a chemical are mixed in the ratio 3 : 7 : 8, the cost price of the mixture is Rs.\ 15 per litre. However, when the same varieties are mixed in the ratio 2 : 5 : 9, the cost price of the mixture is Rs.\ 18 per litre. What is the cost price of the mixture formed (in Rs.\ per litre) when the same three varieties of chemical are mixed in the ratio 6 : 13 : 5?