at least means :
1) one of them OR
2) two of them
[tex] \binom{10}{1} \times \binom{8}{1} \: \: here \: we \: say \: [/tex]
we choose one of them from second graders and another one from first garders
that "or" which I mentioned means we have to use + (plus) to go to 2nd state that I've written
in 2nd we consider that
two of them are second graders in that case:
[tex] \binom{10}{2} \times \binom{8}{0} \: [/tex]
so at last we've got this
[tex] \binom{10}{1} \times \binom{8}{1} + \binom{10}{2} \times \binom{8}{0} = [/tex]
you have to know the formula that I'm using
