A small school bus contains eight first-graders and ten second-graders. if two children are ill, what is the probability that at least one of them is a second-grader?
[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]
