OK you fucks. The ChatGPT spam made me angry, so I looked it up. Here's a good review article on how cocaine works:
They even have a section for poly-drug enthusiasts. Apparently the metabolism of cocaine is very different when it is used with alcohol. Cocaine metabolism has a lot of different paths, and the results are different. Many of the metabolites have a similar molecular weight to cocaine, but there are many different pathways here that make quantity hard to estimate.
Metabolism happens in the liver and other organs, not in the hair, and then the metabolized products are moved around the body. Everything is not conserved during metabolism. It's also known to be unevenly distributed. So yes, if you make wrong assumptions, the calculation is easy.
If you assume all that, you get 150 mg of coke, but the confidence interval on that number is laughably large, and sadly leans toward the higher end.