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. Write an equation you can use to find x, the cost of Rob's lunch.

Robs lunch cost is unknown = x

and we know carla's cost 15 = 15

x + 15 = total lunch cost

now me know 20% is 6

so 20% of total lunch cost is 6

or

(x+15) (.02) which is 20% of lunch and this is 6

or

(x+15) (.02) = 6 this is the equation to find x

this is the answer, but for kicks lets solve it to to find the cost

.2x + 3 = 6

- 3 - 3

.2x = 3

/.2 /.2

x = 15

Robs lunch cost $15 too