Menlo Park Math Tutoring Students Wonder Why the Nines Trick Work!?

Oct 21, 2019 | Palo Alto-Menlo Park
Here at the Menlo Park math tutoring center, Menlo Park math tutoring students regularly use the nines trick. You've probably heard of it. If you haven't, all you need to know is that to use the trick, you put your hands out in front of you and then put the multiplier finger down. For example, if a Menlo Park math tutoring student wanted to multiply 9 by 1, he would put down his first finger (his left pinky) down. Then, the number of fingers to the left is the tens place and the number of fingers to the right is the ones place (in this case 0 and 9). Now we want figure out why this trick works.

Explanation for Menlo Park Math Tutoring Students

To figure out why this works, let's instead, at least for now, think about what happens when a Menlo Park Math Tutoring Student adds nine to a number (we're looking at addition because adding 1 to the multiplier is the same thing as adding 9). When he adds 9, instead of adding normally, let's instead have him subtract 1 from our number and add 10. This is the same as decreasing the ones place by 1 and increasing the tens place by 1. For example, 27 + 9 = 36. But. if the Menlo Park Math Tutoring Student first subtracts 1 from the ones place and adds 1 to the tens place, we get 26 + 10 = 36. The important thing is that these two processes give the same answer! This is exactly what you're doing when you move your finger one space over to the right. We get one more finger in the tens place and one fewer finger in the ones place! In summary, we've seen that increasing the multiplier by one is the same as adding 9, which in turn is the same as decreasing the ones place by one and increasing the tens place by one; since we're doing the exact same thing with our fingers, we get the correct answer!

For More Math and Math Tutoring

For more math practice and math tutoring, make sure to schedule a visit to our center this school year. And for more fun ways to learn math, visit Mathnasium of Palo Alto-Menlo Park. If you're in Palo Alto, and looking for math tutoring services, please don't hesitate to contact us!
 
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