The hay previously cost $15 per bale so 10 bales would cost $15*10 = $<<15*10=150>>150
Double 10 bales is 10*2 = <<10*2=20>>20 bales
Better quality hay cost $18 per bale so 20 bales would cost $18*20 = $<<18*20=360>>360
Farmer Brown would need $360-$150 = $<<360-150=210>>210 more
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