Well,this thing was discovered by me wen i was jus doin tympass wid ma calci...
dunno if anybody knows abt dis...
one thing u will need 2 noe is dat sum of the digits of a number  in lowest form means till its a single digit(eg.65->6+5->11->1+1->2)
Consider 2 numbers,lets say 'a' n 'b'...let 'c' be their sum..the sum of (the sum of (the digits of 'a reduced to its lowest form n the digits of 'b' reduced to its lowest form)) will give u the sum of digits of 'c' in its lowest form...
let me make it simple...consider 356 and 467....their sum is 823....sum of digits of 356 in lowest form is 5 and dat of 467 is 8....5+8=13...reduced form is 4....sum of digits of 823 is 13...reduced form 4.....dey r always equal...
take any 2 no.s and follow dis procedure....it always works....
same thing for multiplication....consider 34 and 56....their product is 1904....
sum of digits of 34 in lowest form is 7 n dat of 56 is 2......product of 7 n 2 is 14....lowest form is 5....sum of digits of 1904 is also 5 in lowest form...works for any 2 no.s...
I'll leave subtraction 2 u...if neone can cum up wid sumthing lyk dis for division, do let me know...
hope u lykd it...