We can use the rule of three to solve this question, first, we need to build our relation, remeber that we need to put calories on one side and ounces on the other:

Now we need to solve it for x, we can build it as a fraction

Now we can solve it for "x", let's do a cross multiplication:
Therefore

We can divide by 12

Now we can just to the calculus and we will have the value of x, if we do it we get

Therefore, there are 295 calories in 20 ounces of drink