Answer: To determine the cost of Rob's lunch, we need to calculate the total amount he paid, including the cost of the meal and the tip.
Given that Carla's lunch cost $15 and the tip amount was $6, we can find the total amount Rob paid by dividing the tip amount by the tip percentage:
Total amount paid = Tip amount / (Tip percentage / 100)
In this case, the tip amount is $6 and the tip percentage is 20%.
Total amount paid = $6 / (20 / 100) = $6 / 0.20 = $30
So, Rob's total payment, including the cost of his lunch and the tip, was $30.
Since we know that Carla's lunch cost $15, we can subtract this amount from the total payment to find the cost of Rob's lunch:
Cost of Rob's lunch = Total amount paid - Carla's lunch cost = $30 - $15 = $15
Therefore, the cost of Rob's lunch was $15.