Find the GCF (Greatest Common Factor) in Your Head | Fast Trick for Math Competitions

What's the biggest number that divides both 18 and 66? No listing needed! Get close, subtract, and check. Siggy's subtraction trick finds the greatest common divisor fast. 0:00 18 and 66: biggest shared divisor? 0:17 Trick #28: the GCD 0:26 GCD of 16 and 56 0:55 Your turn: 27 and 63 1:15 Why it works 1:27 Not done? Go again: 18 and 66 1:54 Boss level: 96 and 264 2:30 Your challenge 2:40 Next time: LCM Comment your challenge answer! Subscribe to Math Sigmas for a new trick every week. #mentalmath #mathtricks #mathforkids