We’re all taught that “you need at least as many equations as you have variables.” In fact, that’s one of the first principles in a lot of GMAT texts.
The simple fact that it’s in the damn GMAT books is almost undoubtedly why tricky DS questions love to punk you by having a variable simply disappear as part of the structure of the question.
In short, when you reorganize the information given at the beginning of the question (and occasionally when you plug a Statement in–but not in this video), you’ll find that what you thought was a variably really just falls out.
Why was there a variable there in the first place if it wasn’t necessary?
Look, this is like a three-year-old asking “where the sun sets.” It begs a longer, philosophical discussion of a “placeholder variable,” but mercifully I won’t bother you with the details.
After all, this is GMAT-land, and getting the job done is more important than the dreaded “whys.”
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PPS I mean if you really do care to hear about placeholder variables, let me know and I’ll explain further. I’m geeky like that.