Incorrect test case

Please correct me if I'm wrong.

The second self-testing assert is incorrect. If tin:iron = 1/2 and iron:copper = 1/2 then tin:copper = 1/4 but test says tin:copper = 2/1 (copper:tin = 1/2 which is the same)

Also there is no any ratio with gold that means the test is unsolvable either way.