(1), the total revenue of producing 20 sets. When q=20, R(20)=20*20-( 1/5)*20? =320。 Average income =R(20)/20=320/20= 16.

(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.