There's at least one solution that passes all existing tests but would fail this valid test, please consider adding the following test:

Test.assert_equals(validate_word("abccabcd"), false)

Use more descriptive parameter names in the beginning code template. list_ and n could be more descriptive and the underscore suffix is unconventional for Ruby. Suggestion: words and sort_char_position as clearer param names. Good for participants to see good naming practices in the provided code.

Clarify description of handling negative floats

Consider adding a test for the case when no arguments are given. Handle by returning 0 or throwing an ArgumentError.

describe "sum" do
  it "returns 0 for no arguments" do
    Test.assert_equals( sum, 0 )
  end
end