Okay, so here's a hypothetical example. My 36 card deck is:
9x time factory
9x time pendulum
6x hope
6x flooding
6x rustler
18/36 of my cards are time, thus fulfilling the 50% rule for cards of your own element.
The deck I want to actually play is:
9x time factory
3x time pendulum
6x hope
6x flooding
6x rustler
Which is a 30 card deck, but it only uses 12 time cards. Could I legally do this for round 9?