Asmall 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?

At least means :
1) one of them OR
2) two of them


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:

so at last we've got this


you have to know the formula that I'm using