The expression 15r + 15w gives the total time Mario spends exercising. The variables r and w give the number of minutes spent per day running and walking, respectively. Which expression could also be used to find the total time Mario spends exercising ?

Acar started driving from city a to city b, distance between which is 620 miles, at a rate of 60 mph. two hours later a truck left city b and started moving towards city a at a rate of 40 mph. how many hours after the truck's departure will the two meet?