[suwarnapatel]

Take time for each machine to complete Job = X
Therefore Rate of each machine = 1/X

When 2 Machines Work Together:

1/X + 1/X = 1/K
2/X=1/K
X=2K

Similarly
When 3 Machines Work Together:

3/X = 1/(K-2)
3/2K=1/(K-2)

3K-6=2K
K=6