Final answer:
To find the total cost including a 20% tip, multiply the cost of the lunch by 20% and add it to the cost of the lunch.
Step-by-step explanation:
To find the total cost including a 20% tip, you first need to calculate the tip amount.
Multiply the cost of the lunch by 20% to find the tip: $15 x 20% = $3.00.
Next, add the tip amount to the cost of the lunch to find the total: $15 + $3.00 = $18.00.