R↙from historical "rent" = profit P = priceQ = quantity TC = total cost K = capital stock

r = profit rate g = growth rate ← a rate is a percentage

Y = income = PxQ
R = Y - TC
r = R/K
r = g × (1 - x ↱ external funding (percentage) eg. 0.75 (internal 0.25) ) / sc
← ratio of retained earnings to gross profit (out of the profit, the firm keeps eg 0.5 to finance expansion)