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You’re prepared to fight for your survival through the zombie apocalypse, but are you ready to solve math to save your friends?

This animated brain teaser from the folks at TED-Ed puts that question to the test because you have to get your three friends across the bridge in order order to save them.

One small catch, there are four of you and only two people can cross the rickety rope bridge at a time and, of course, some people move slower than others (dammit professor!).

Tell us in the comments how long it takes you to solve the riddle.