So,
Andy paid $20 total for his lunch (100%).
15% is for drink.
Therefore, 100 - 15 = 85% is the percent that was constituted by the food.
85% of 20 is equal to 0.85 * 20 is equal to:
[tex] \frac{17}{20} *20= \frac{17*20}{20} = 17[/tex]
Andy's food cost $17.