Carla and rob left a 20% tip after having lunch at a restaurant the amount of the tip was $6 carla's lunch cost $15 which equation can you use to find x, the cost of robs lunch

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.

6=0.2(x+15)