Question 1000927
Imagine that the field has an area A given in square feet (the specific unit doesn't matter but it will help understand). Each tractor will cover the field at a different rate (because they operate at different speeds, etc) expressed in square feet per day. For example, 500 square feet/day means the tractor would only complete 500 square feet in one day. If we divide the area of the field A by the rate covered by a given tractor, we get the total time it takes to process the entire field.

For tractor 1:

A/rate1 = 4 days                    

For tractor 2:

A/rate2 = 6 days

Quick dimensional analysis check: square feet / (square feet / day) = days 

Express the equations in terms of A, the quantity that relates to both events.

A = 4 days x rate1 
A = 6 days x rate2

Now, the area A is the same so we can do:

4 days x rate1 = 6 days x rate2  
rate2 = (4/6) x rate1 

rate2 = (2/3) x rate1

Thus, we now know how the performances of the tractors relate to each other.

If both tractors work together, the number of days is still the area of the field divided by the rate. However, this time, you use two rates!

time for combined performance = A / (rate1 + rate2)
                              = A / (rate1 + (2/3) x rate1)
                              = A / ((5/3) x rate1)

                              = (3/5) x (A/rate1)

Remember that A/rate1 = 4 days, so we get:

                              = (3/5) x 4 days = 2.4 days

There's an equation you can use or memorize to work such problems but it's better to know what's going on.