let A = { 2,4,6,8,9,10 }   and B = { 1,2,3,4 }

then number of onto functions from A to B is :

plzz. explain

No. of onto functions from set A(containing n elements) onto a set B(containing r elements) is equal to the no. of ways of dividing n  things into r groups so that no group remains empty if n>=r  and zero if n<r.

The formula is rⁿ -^rC₁(r-1)ⁿ+^rC₂(r-2)ⁿ+.....+(-1)^{r-1 r}C_r-1

Here in your question n=6 and r=4 Substitute them.You will get the answer.