Sally would have to drive 180 mi in a work week. You multiply the amount of miles it takes her to get to her job by how many times she drives to and from work a day. (She drives 18 mi to work and and 18 to get home) (18•2=36) So she drives 36 mi in one day. Then you multiply the amount of miles she drives per day (36) by how many days she works a week (5) and you get 180 mi in a work week.
