(2) Marginal income △ r (q) = r (q)-r (q-1) = 20q-(1/5) q? -[20(q- 1)-( 1/5)(q- 1)? ]=20-(2q- 1)/5 .
△ r (20) = 20-(2 * 20-1)/5 =12.2. Its economic significance is that the total income will increase with the increase of output per unit.
For reference.