let
x--------->number of products
y1--------> the cost of producing a product
y2------> the cost of sell a product
we know that
y1=120,000+20x-------> equation 1
y2=50x------> equation 2
equate equation 1 and equation 2
120,000+20x=50x
50x-20x=120,000
30x=120,000
x=4000
the answer is
4000 pieces