There's a certain mind that either doesn't realize they're sidestepping the problem and turning it into a editing review, or realizes it, and doesn't understand why it seems off-topic/trivial to others.
What's especially strange here is, they repeatedly demonstrate if you interpret it that way, the problem is obviously, trivially, unsolvable, in a way that a beginner in algebra could intuit. (roughly 12 years old, at least, we started touching algebra in 7th grade)
I really don't get it.
When I've seen this sort of thing play out this way, the talking-down is usually for the benefit of demonstrating something to an observer (i.e. I am smart look at this thing I figured out; I can hold my own when the haters chirp; look they say $INTERLOCUTOR is a thinker but they can't even understand me!), but ~0 of that would apply here, at least traditionally.
Buying two of the items gets you the third for free.
The implication is any two.
It’s ok that it’s ambiguous. It happens. In most cases, we clarify and move on. There’s no need to defend it.