Question 90
Let $ABCDA'B'C'D'$ be a cube of edge length $1$. Points $P$, $Q$, $R$ and $S$ are given on the edges $\overline{AB}$, $\overline{AD}$, $\overline{C'D'}$ and $\overline{B'C'}$, respectively, so that $PQRS$ is a square whose centre is in the centre of the cube. What is the length of the side of the square?
