The questions gives the costs (or information to calculate the cost) of both A and B.

The questions asks Cost of 1 Machine B compared to Cost of 2 machines A.

Cost of machine B
50000 + (50000-20%of50000)*40% = 50000+(50000-10000)*.4 = 66,000
Cost of machine A
20000 + (20000-20%of20000)*40% = 20000+(20000-4000)*.4 = 26,400

now keep in mind that we need 2 A machines

66,000 - (2*26,400) =
66,000 - 52,800 =
13,200

answers your question.