This is a question of proportionality. However, it is not direct proportionality as it is expected that as the number of tractors increases, the work is finished faster as opposed to fewer number of tractors. This is referred to as inverse proportionality.
Therefore;
6 tractors ----- 10 days
18 tractors --- x days
Then,
x = (6*10)/18 = 10/3 days = 3 days, and 8 hours.
This means that 18 tractors will take 3 days and 8 hours to collect the same harvest.
