$11.30
Explanation:
First, convert the 20% to an actual number that can be used in a calculation.
For percents, this is always done by simply dividing the percent (in this case 20%) by 100%.
So, the conversational term "20%" becomes 20% / 100% = 0.20 in terms of a real mathematical number.
Second, you need to find out what 20% of your $56.50 meal cost is. This is always done by multiplying 0.20 by $56.50, or
0.20 x $56.50 = $11.30
So, the amount of tip you are going to leave is $11.30.
This makes the total cost of your meal (to write on your charge slip or other payment)
$56.50 + $11.30 = $67.80.