Share or comment on this article: Easy-looking math equation mystifies people as they forget 'special rule' - can you solve it in 30 seconds?

A simple chalkboard sum has tripped up thousands - and the right answer depends on remembering one classroom rule you probably haven't used since middle school.

In a boon to algebra students everywhere, a professor at Carnegie Mellon University has devised a simpler and more efficient way to solve problems involving the quadratic equation.