http://easycalculation.com/what-is-hcf.php
http://www.mathsteacher.com.au
http://www.lazymaths.com/speed-math/
http://mental-maths-trick.blogspot.in/2011/10/do-super-quick-maths-calculation-using.html
Ratios Calculation:
http://www.mathsisfun.com/numbers/ratio.html
Short cut:
http://www.jainmathemagics.com/page/5/
Calculating Square of numbers quickly…
Lets calculate the square of 54
So (54)^2 = 5^2 +4 — 4^2 = 25 +4 —-16 =29——-16= 2916
Similarly (55)^2 = 5^2 +5 –5^2=25+5——25=30———25= 3025
Similarly (56)^2 = 5^2 + 6–6^2=25+6——36= 31——–36= 3136 etc..