Let E={1,2,3,4} and  F={1,2}.  Then the number of onto functions from E to F is

Since each element of E can be associated with elements of set F in two ways. So, the total number of functions from E to F is  2×2×2×2=16.  Out of these functions, the functions, which are not onto are f(x)=1∀x∈E and  f(x)=2∀x∈E.  Hence, the number of onto functions =16 -2=14.