There are three possibilities to choose a boy (i.e C3 1).
Because there is only one boy, there are two kinds of other girls (C2 1).
The above is gradual, so any boy has 3*2=6 kinds.
So the probability of just a boy is 6/ 10=3/5.
The opposite of at least one boy is that there are no boys, that is, all girls.
There are only two girls in a room, so there is only one possibility.
So the probability of at least one boy among the participating students is:
1- 1/ 10=9/ 10