One less than twice a number is seventeen.
Since you have to mix all of it together, the whole cocktail is 20+10+15=45.
And the portion that is coconut milk is 15ml. So that means there is 15 out of 45 or 15/45 coconut milk in the cocktail. Then the simpelest form is 1/3.
I have done this question yesterday it is 1 and 3