Two dozens of pencils are equal to 2 x 12 = <<2*12=24>>24 pencils.
So the total cost of pencils is $0.5 x 24 = $<<0.5*24=12>>12.
The total cost of folders is $0.9 x 20 = $<<0.9*20=18>>18.
Thus, the cost to buy the office supplies is $12 + $18 = $<<12+18=30>>30.
#### 30