Riddle: The fastest time Mary can wash her car is 4min, while the fastest her husband Tom can do it is in 6min. What's the fastest time they can wash it if they did it at the same time?
Answer: Answer= AB/A+B= 4*6/4+6= 24/10= 2min 24sec
It takes Mary 2min 24sec to wash 60% of the car
It takes Tom 2min 24sec to wash 40% of the car
So it takes them 2min 24sec to wash 100% of the car together.