Calculate the number 4020

So one day, I gynecologist got bored with his job. But he realized that he had spent many years training with his hands, and he decided he would switch to careers to something else that he could use his hands with. Not wanting the stress of being a medical professional, he decided to attend vocational school to learn to be an automotive mechanic. He worked really hard and studied day and night. When the results of his final exam came in, he was quite perplexed. It showed that he got 150% on the test. The doctor figured this had to be a mistake so he called his instructor at the vocational school.

Doc: there must be some mistake. It says I got a 150% on the final exam. Could you explain that?

Instructor: well, for the first part of the test you took apart the cars engine perfectly. That counted for 50 points of your test. Then you went and put the engine back together perfectly. That was another 50 points. But those last 50 points? Well that’s because none of us have ever seen anyone do it through the muffler before.