Nice explanation, singh181!
One more thing to add in response to tonebeeze's initial question:
When applying your knowledge of idioms you can eliminate choices that clearly misuse an idiomatic expression. You cannot choose a choice simply because it uses an idiom correctly; similarly, you cannot eliminate a choice because it does not use your preferred idiom.
This question is a good example. Yes, in a countable/uncountable situation you'd want to use "number", and not "amount", for a countable noun. But "figure" is a synonym for "number" - "figure" is not wrong! You could eliminate "amount" if you want, but that doesn't mean that "number" is definitively correct. There may be another correct way of phrasing the same thing other than the idiom that you've studied.
One of our instructors told me that "the trickiest idioms I've learned on the GMAT I've learned by taking the GMATPrep tests", noting that "once I've eliminated all the wrong answers there's sometimes an idiom left standing that I never really knew before". The English language is huge - there are several ways to phrase any given thought (there are also multiple ways to do so; countless ways, even; you could even say that each thought could be expressed in any variety of ways). Please, please don't get in the habit of looking for one particular idiom - Sentence Correction is much more a test of your ability to eliminate what you know to be wrong than it is of searching for what you know to be right.
Brian Galvin
GMAT Instructor
Chief Academic Officer
Veritas Prep
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