6 tractors take 12 days to collect the harvest. how long would it take for 18 tractors to do the same amount of work?

12 day harvest needs 6 tractors
1 day harvest needs 6/12 tractors = 1/2 tractor ?
(better, 2 day harvest needs ((6/12)*2) = (12/12) = 1 tractor ?)
18 day harvest needs ((6/12)*18) = ((1/2)*18) = 9 tractors