It's based on mass to charge ratios. Methyl is 15 when it has a -1 or +1 charge. Mass spec usually does not tell you everything you need to know because a lot of fragments have similar m/z ratios. Usually they give you an IR or NMR to go with it. There are also books and probably webpages that give you some common fragments and they're m/z. Another helpful thing is to look for multiples, like if you see a 15 for methyl you might want to look for CH2, which would be at 7.

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