Each ml of ink costs 50 cents so 20 ml costs 20*50 = <<20*50=1000>>1000 cents
There are 2 whiteboards in each class so each class needs 2*1000 cents = <<2*1000=2000>>2000 cents
There are five classes in the block so they all need 5*1*2000 cents = <<5*1*2000=10000>>10000 cents
100 cents make a dollar so 10000 cents make 10000/100 = $<<10000/100=100>>100
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