If #6# tractors take #6# days to collect the harvest, then how many days will it take #18# tractors to collect the harvest?
Assuming they work at the same rate and are in direct proportion:
This is an example of inverse proportion.
Where there are two quantities and one increases, the other will decrease. As there is a task to be completed (the harvesting), the MORE tractors that are doing the work, the LESS time it will take to do the job.
If there are