Answer:
4,167 units
Step-by-step explanation:
The computation of the minimum number of units that should be painted is as follows;
Let us assume the number of units in 3 years be x
 So the labor cost without the machine is 1.4x
And, the Labor cost with the machine is 0.2x + $15000 
 Now the equation is 
1.4x = 0.2 x + $15,000 
 x = $12500 
 Now 
For Each year, it is 
= $12,500 ÷ 3 
= 4,167 units