Learning Ruby, Looking for Possible Refactoring for If Statement

I've got a few ideas but wanted to get a few other options. Here is the ugly:

    def normal_balance
       if type.to_s == 'Asset' or type.to_s == 'Expense'
         if contra
           "Credit"
         else
           "Debit"
         end
       else
         if contra
           "Debit"
         else
           "Credit"
         end
       end
     end
 

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