Bubba can mow a field in 8 hours. if skeeter , they both can do it together in 3 hours. how long would it take skeeter to mow the field by himself?

When they work together, they do  1/3 of the job in one hour.

                   (Bubba) + (Skeeter) = 1/3 job/hour    

Subtract 'Bubba'
from each side:            Skeeter  =  (1/3 job/hour) - (Bubba)

Bubba, working alone, does  1/8 of the job in one hour.

                                      Skeeter  =  (1/3 job/hour) - (1/8 job/hour)

Can you subtract 1/8 from 1/3 ?

Before you can subtract, you'll need a common denominator.
I've got a nice one here . . .  24 .

                   1/3  =  8/24
                   1/8  =  3/24                                                         

                   Skeeter  =  (1/3 job/hour) - (1/8 job/hour)

                                  =  (8/24 job/hour) - (3/24 job/hour)

                                  =        5/24 job/hour .

Skeeter mows at the rate of  5/24 field/hour.

The other way to look at that is:     24/5 hour/field  =  4.8 hours .

Check:

         Bubba:     1/8     field per hour  =  0.125
         Skeeter:  1/4.8  field per hour  =  0.208333...     

         Together:                        (0.125 + 0.208333)

                                             =  0.333... field per hour

                                             =    3 hours/field .                  yay!