An Active Member is a player who has played 5 games
A clan that has x members needs 5(x/2) games to be played

Games needed to be played / Active Member's games played = Half of the clan's members
5(x/2) / 5 = x/2


Even simpler:
A city has 30 workers (a clan has 30 members)
The city divides these workers by 2, giving us 15 workers (a clan divides those members by 2, giving us 15 members)
The city needs 75 houses built (a clan needs 75 games to be played)
A hired worker can build 5 houses (an active member has 5 games played)
The city has to hire at least 15 workers, because if each of the worker's makes 15 houses, we have exactly 75 houses (the clan needs at least 15 active members because if each of those members plays 5 games, we have exactly 75 games played)

So in the end, the city divided it's workers into two, giving us 15, and they also had to hire 15
So in the end, the clan divided it's members into two, giving us 15, and they also had to have 15 active members