If it took Hadassah six hours to paint 12 paintings, then she paints 12/6 = <<12/6=2>>2 paintings in an hour.
If two paintings take an hour to complete, 20 paintings will take 20/2 = <<20/2=10>>10 hours.
The total time she took to finish all the paintings is 10+6 = <<10+6=16>>16 hours.
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