Sorry, if I confused things. If it is worth doing 20 x-rays to diagnose 1 fracture, then you should x-ray anyone with a probability of fracture > 1/20 = 5%. If it is worth doing 50 x-rays to diagnose 1 fracture, then your Ptt = 2%.
Apparently my earlier post made some people who were doing the problem correctly think that they were doing it wrong. They looked at 1 - NPV, which for Part C was 1.6% and for Part D was 3.5%. This is post-test probability after a negative test. If your treatment threshold is 5%, then it doesn't matter whether you use the numbers from Part C or D. A negative elbow extension test allows you to skip the x-ray. But if your treatment threshold is 2%, then it does matter. If you use the numbers from Part D, a negative elbow extension test does NOT allow you to skip the x-ray.