Australian AMC 2009 - Senior Question 11

Question 11

$QRST$ is a square with $T$ at $(1,0)$ and $S$ at $(2,0)$. Which of the following is an equation of the line through the origin which bisects the area of the square?

Question illustration

$y=\frac12x$
$y=\frac13x$
$y=\frac23x$
$y=2x$
$y=3x$