There are three scenarios to consider ( C(n,r) = n!/((n-r)!r!) ):
1. You can choose 3 boys and 2 girls: C(7,3)*C(6,2)
2. You can choose 4 boys and 1 girl: C(7,4)*C(6,1)
3. You can choose 5 boys and 0 girls: C(7,5)*C(6,0)
Calculate each result and add them (they are mutually exclusive, so you can add them without overcounting).