In 8 minutes they can wash 2 and ⅓ cars (Al finishes his car and gets (8-6)/6 or ⅓ of the way into a third car). Round off 2 and ⅓ to 2.333 cars in 8 minutes. Therefore the time to wash one car working together is 1.166 in 4 minutes. So less than four minutes.
So, assume the work is 6*8 = 48 units,
then Person #1 works @ 8 units/min
Person #2 works @ 6 units/min
together they work @ 14 units/min
time needed = 48/14 = 24/7
= 3 3/7 min
