fuel mix?
I'm not an expert, but logic would state that if you mixed equal quantities you will end up with 15% nitro.
e.g. 1 litre 10% has 100ml nitro in it, 1 litre 20% has 200ml nitro.
Therefore 2 litres of mix will have 300ml nitro, 1 litre mix would have 150 ml nitro, so 15%.
Also, increasing the nitro from 10% to 15% will mean greater performance thats why the higher the nitro content, the more expensive the fuel.