more general:
owing to increased production, the 
real labour cost increase (i.e. the cost of
labour in terms of goods produced)
Real labour cost: Y-P       We assume  P=?Y
=approximately equal to the labour cost
for the old equipment, because that is by far
the greater part. 
The increase in the real labour cost
corresponds to the the fall in profits of the old
equipment (increased wage is decreased price!)
That must be equal to the gain of new equipment



given the total profit P