Received a PM asking me to respond.
People say "plan on" all the time. We also use all kinds of other grammatically incorrect stuff every day, but the GMAT doesn't let us get away with it.
Here's my rule for studying idioms: if I haven't seen it in an official question, it's not high on my priority list. Why? Because there are thousands and thousands of idioms in the language, and I can't study them all. I'm most interested in studying the ones that have actually been tested on the exam.
This one (although it is one of ours, yes) does not follow my rule. So it's not high on my list of things to worry about.
(By the way, how do we know what the test has actually tested? The idioms chapter in the 3rd and 4th editions of our SC strategy guide was developed from official question sources only. That doesn't mean it covers every idiom ever tested on the GMAT - but every idiom on the list was officially tested at some point. You can add to the list yourself as you do questions from official sources and see other idioms you didn't know.)
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Stacey Koprince
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Manhattan GMAT
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