Set derived on mortgage amount

app/models/form/sales/questions/mortgage_amount.rb

@@ -18,4 +18,8 @@ class Form::Sales::Questions::MortgageAmount < ::Form::Question
     2023 => { 1 => 91, 2 => 104, 3 => 112 },
     2024 => { 1 => 92, 2 => 105, 3 => 113 },
   }.freeze
+
+  def derived?(log)
+    log && log.mortgage_not_used?
+  end
 end

spec/models/form/sales/questions/mortgage_amount_spec.rb

@@ -28,7 +28,7 @@ RSpec.describe Form::Sales::Questions::MortgageAmount, type: :model do
   end
 
   it "is not marked as derived" do
-    expect(question.derived?(nil)).to be false
+    expect(question).not_to be_derived(nil)
   end
 
   it "has the correct hint" do
@@ -46,4 +46,20 @@ RSpec.describe Form::Sales::Questions::MortgageAmount, type: :model do
   it "has correct min" do
     expect(question.min).to be(1)
   end
+
+  context "when the mortgage is not used" do
+    let(:log) { create(:sales_log, :completed, mortgageused: 2, deposit: nil) }
+
+    it "is marked as derived" do
+      expect(question.derived?(log)).to be true
+    end
+  end
+
+  context "when the mortgage is used" do
+    let(:log) { create(:sales_log, :completed, mortgageused: 1) }
+
+    it "is marked as derived" do
+      expect(question.derived?(log)).to be false
+    end
+  end
 end