to summarize, this test was about filial piety
there are two piles of cards - one of "children" cards and one of "parent" cards
each person is only allowed to hold one card
"children" must make a filial offering (gift) to the "parent"
wang lu purposefully interpreted this as whoever has the "parent" card will fail, because only "children" are allowed to make filial offerings
"parents" are unable to make filial offerings, and so cannot express filial piety
he purposely made it into a battle of who can acquire the "children" cards, which shengjing sect largely succeeded in doing
however, the test isn't actually that simple
there is a chinese saying that "having no posterity" is the worst way of being unfilial. that is, not bearing children and ending the family lineage
when shengjing sect confirmed that they were done, they as "children" did not become "parents" and raise other "children", hence they were unfilial and they failed
the actual correct solution would be to form a chain
a "child" would give a gift to a "parent", then flip their card over to become a "parent", and then receive a gift from another "child"
this would only cause two people to fail:
1. the first person of the chain, "parent", did not have a "parent" to give a gift to and so is unfilial
2. and the last person of the chain, a "child", is unable to raise another "child" as a "parent" and so is also unfilial