"1" based on the original price of 20 units.
540( 1-85%)= 3600 yuan ... The unit "1" is the original price of 20 units.
3600÷20 = 180 yuan ... the original price of each set.
180 ÷ (1+20%) =150 yuan ... the original purchase price of each set.
9000÷ (180-150) = 300 ... the number of these fans.