Answer:
We conclude that 86.9% cost food.
Therefore, we conclude that 100% - 86.9% = 13.1% is the tip cost.
Explanation:
We know that the customer paid a total bill of $23 at a restaurant.
Of that amount, $20 was the cost of the meal and the rest was the tip.
We use the following proportion:

We conclude that 86.9% cost food.
Therefore, we conclude that 100% - 86.9% = 13.1% is the tip cost.