Hi nrxbra001,
First off, a 640 is a solid score (it's right around the 80th percentile overall), so it could be enough to get you into Cambridge. If you know how to calculate an average, then you know that not everyone who got into Cambridge scored 690 (some scored higher, some scored lower). As such, a retest might not be necessary. You're ultimately asking Admissions questions though, so you would likely find it beneficial to speak with an Admissions Expert about your overall profile. There's a Forum full of them here:
https://gmatclub.com/forum/ask-admission ... tants-124/It takes time to put together a well-crafted application, so If you have not actually done any of that work yet, then you should plan to apply at a later date (and not 'rush' to make a March 10th deadline).
GMAT assassins aren't born, they're made,
Rich