You can put this solution on YOUR website! How many ways are there for 15 boys and 15 girls to pair up into 15 different heterosexual couples?
If you have 2 boys (ab) and 2 girls (AB) how many pairings are there?
Each boy can pair with each girl.
Ans: 2*2 4 pairs
Carry that logic to 3 of each or 4 of each or 15 of each.