let 5 girls be A,B,C,D and E.
Combinations against each other will be found by matching from 1st girl downwards, but take note not to repeat:
A= AB,AC,AD,AE, A girl has exhausted evryone, we move to next,
Hence 10 matches played.
and you can notice the pattern going downwards for next time calculation!!! (4+3+2+1+0) = 10