Given:
adds one more worker, total monthly revenue will increase from 54,000 to 66,000
adds one more tractor, total monthly revenue will increase from 54,000 to 72,000
additional worker costs 6,000 per month
additional tractor costs 6,000 per month
Marginal Revenue of labor = 66,000 - 54,000 = 12,000
Marginal Revenue of capital = 72,000 - 54,000 = 18,000
MRPL/PL = 12,000 / 6,000 = 2
MRPK/PK = 18,000 / 6,000 = 3