This question is a little tricky. Carla had a bill of $15, which we will add to the cost of Rob's lunch. They left a 20% tip on the TOTAL cost of both their lunches, which should equal $6. 20% is equivalent to 0.2. So, the equation is 6=0.2(x+15). In other words, 20% of the sum of Carla and Rob's lunches is 6 dollars.
Answer: 6=0.2(x+15)
