A teacher is trying to pick 7 students to represent his class at an upcoming state competition. If there are 18 boys and 10 girls in the class, in how many ways can the teacher pick 4 boys and 3 girls to attend the event?
Number of ways
=(18 choose 4)*(10 choose 3)
=C(18,4)*C(10,3)
with C(n,r) as defined before.