3 cars every 2 hours is the same as 3/2 = 1 1/2 cars every hour

one car takes 2/3 of an hour = 40 minutes to complete

a) 6 cars take 4 hours

b ) 9 cars take 6 hours

c) 12 take 8 hours

d) 4 take 8/3 hours or 2 hours and 40 minutes

e) 8 take 16/3 hours or 5 hours and 20 minutes

f) 16 take 32/3 hours or 10 hours and 40 minutes