test/fixtures/credit_card_bills.yml

@@ -1,9 +1,5 @@
 # Read about fixtures at https://api.rubyonrails.org/classes/ActiveRecord/FixtureSet.html
 
 one:
-  description: MyString
-  amount: 9.99
-
-two:
-  description: MyString
-  amount: 9.99
+  description: Credit Card Bill One
+  amount: 1_000.00

test/models/credit_card_bill_test.rb

@@ -1,7 +1,9 @@
 require "test_helper"
 
 class CreditCardBillTest < ActiveSupport::TestCase
-  # test "the truth" do
-  #   assert true
-  # end
+  def test_unpaid
+    credit_card_bill = credit_card_bills(:one)
+
+    assert_equal 870.00, credit_card_bill.unpaid
+  end
 end