i can share a few. Hope you would find them useful.

1. FIND SQUARE ROOT:
to find a square of a 2 digit number ending in 5 simply sqaure teh 5 and multiply the digit preceding 5 by adding 1 to it. e.g
45=4*5 and square of 5 is 25 so answer would be 2025
75 squared equales 8*7 and 25 equals 5625


2. SOLVING FRACTIONS:
to solve fractions, first make the denominators of the fractions equal by multiplying and dividing by a suitable number. then simply add the numerators. e.g

1/2 +7/8 multiplying 1/2 when multiplied and divded by 4 becomes 4/8 so the answer will be 11/8

SQUARING A DECIMAL:
(0.002)^2

first square 2 that is 4 and double the number of digits to the right of decimal.
(0.002)^2=0.000004
(0.003)^3=0.000027


to multiple any number of a decimal, don't waste ur time doing the laborious work: for example (1.3/4)*200/13 would appear to be a heck of a difficult question for someone who doesn't know the trick. simply convert 1.3 into fraction by shifting the decimal once place right to it.

(13/10*4)*(200/13) after cancelation the answer would be 5


These are few basic quick math tricks. keep practicing these cuz practice makes man perfect and it's likely that you might even develop your own tricks that might help you getting the most complex calculation done within seconds