While I suppose the assert_equal statement is technically correct, I'm surprised you didn't get an error for the assignment of the 'expected' variable, having left the value for "jenny" as '__'.  The answer that would have demonstrated that you understood how the merge method works would have been:

expected = { "jim" => 54, "amy" => 20, "dan" => 23, "jenny" => 26 }
assert_equal true, expected == new_hash