From MitDavidDv: Indian Vedic Mathematics

I'd not care if anybody does offer short cuts to calculation, if my focussed objective was to attack GMAT questions. Vedic mathematics be a favorite pass time, when you do have the time. I love the Lilavati Ganitam (an ancient Indian treatise) as a favorite pass-time. Spending time on basic concepts would be more fruitful.

That said, I do use some shortcuts when l see numbers on the GMAT questions. More often than not, I have the following numbers on my finger tips:

sqrt(10) ~ 3.15
sqrt(2) ~ 1.41
sqrt (3) ~ 1.73
sqrt(5) ~ 2.25
sqrt(7) ~ 2.65

I also use extensively the following rule:

The square of any number in a form x5 is x(x+1)25.

A little explanation for the above rule:

325*325 = (32*33)25=105625

This is what I used to compute sqrt(10) above. 1000 is nearly 32*31 so sqrt(10) should be b/w 32/10 and 31/10 or nearly 3.15. The result may not be the most accurate computation on earth. But GMAT does not require you to be more computative than this. At least, that is what I have observed. Others can correct me if I am wrong.

Regards
Rahul