Q A= {1,2,3,4} B={1,2,3} What is max no of onto functions from A to B. (Give the working)
ans . total no of functions possible from A to B = 
but this also includes those functions in which 2 required images are left out + 1 required image if left out !
those functions in which 2 required images are left out = 3C1 x 1 x 1 x 1 x 1 = 3 --------------- (1)
those functions 1 required image if left out = 3C2 x 2 x 2 x 2 x 2 = 16 x 3 = 48 --------------- (2)
but (2) already includes case (1)
total no of required functions = 81 - 48 = 36