2C6H6(l) + 15O2(g) --> 12CO2(g)+ 6H2O(g) how to calculate the ratio?

Answer 1

Ratio of what? Ratio of mols of one to mols of another. The coefficients in the balanced equation does all of that for you.
In this equation, 2 mols C6H6 = 15 mol O2 = 12 mols CO2 = 6 mols H2O.
So you can convert from mols of one substance to mols of another easily. For example, suppose we want to know how many mols O2 it will take to completely react with 10 mols C6H6?
10 mols C6H6 x (15 mols O2/2 mols C6H6) = 10 x (15/2) = 75 mols O2 required.
If I've guessed wrong about what you want repost here and I can follow up.

Answer 2

look it up