For this problem, we will assume that each painter paints the house at the same rate. Furthermore, we will use the equation: (# of houses painted)=(rate)(time). Therefore, we can write the following for 3 painters where 'r' is the rate of one painter:

1=(r+r+r)(3)

1=9r

r=1/9 house/day

Thus, we have found the rate of each painter. Now we can write the following for 2 painters where 't' is the time it takes 2 painters to paint 1 house:

1=(1/9+1/9)t

1=(2/9)t

t=9/2 days

We can also write 9/2 days as 4.5 days.