27 The ones that have the knifes have proven themselves to be better fit then the other, this test is just to see if they are good enough to join.
*At this point everyone is allowed to open the chest they hold and they all do it at the same time.
It's clear that they have not been told about this last test as people are confused. The ones where their chest was empty need to remove their mask and look at the other of the pair.
The ones that got a knife now hear they need to kill the other. 15 people do so without hesitation, some give a quick death, others a not so quick one. The others start to cry or start to yell but eventually mostly thanks to others do it. The 5 that did not do it with hesitation need to stay, the others need to go.
When everyone that had to leave is gone, 5 people that stood at the side come to the 5 children and cut their throat through. More people come to clean up the bodies*
15 of the 20, better then I expected to be honest.