Today's school methods are rote etc. developing tunnel vision - none of which is desirable. The same school methods (along with GENUINE & ARTIFICIALLY CREATED FORMULAE!) are dished out with great slogans and promises as aptitude development stuff.
Mugging up formulae and methods, drilling a 1000 times and vomiting answers at supersonic speeds
is not aptitude development!
Aptitude is thinking and reasoning out of the box and not by rote memory nor uni-directional thinking nor traditional thinking.
These are mental calculations done in a couple of seconds - once your mind is trained, of course.
Remember we need to calculate 12 %. That is all.
2) Of 840, 10% is 84; 2% is a fourth of 84, i.e. 21; so discount (total) is Rs.105.
3) 100% >> 50% >> 25% >> 12%; So halve 840 four times; 840 >> 420 >> 210 >> 105; so Rs.105.
4) 12% of 840 is simply 'one-eighth' of 840; divide 840 by 8; so is Rs.105.