One of the initial stumbling blocks in doing mental multiplication is you might forget that multiplying larger numbers requires breaking those larger numbers into more manageable pieces.
For instance, if you see 17 x 6, you might think I can’t multiply 17 times anything, except maybe one or two. But the thing is you don’t want to think of 17 x 6 as multiplying 17 times anything. I know, kind of crazy, right? But here’s how you do it:
You break up 17 into two manageable numbers that you can both multiply times 6. Then, add the result of those two numbers to get the answer. Sure, you could 9 and 8, meaning (9 x 6) + (9 x 8), but the easiest way is to choose a number ending in a zero, since those numbers are easiest to multiply.
For example, 17 can be broken into 10 and 7. (Remember: 7 + 10 = 17). Multiplying the 6 by 10 should be pretty instantaneous: it equals 60. That leaves the 7, giving us 7 x 6 = 42. Adding those two gives us 42 + 60 = 102.
An important point is not to think of this as a writing exercise, in which you write 10 x 7 and 6 x 7. Multiplying these out the traditional way—pencil and paper—would work best. But when doing numbers in your head, breaking them up this way makes things a lot easier. What you are doing is multiplying the 10 x 6, then keeping that 60 “on hold”, while you multiply 6 x 7. Then you add that 60 to the 42.
What happens when you have a slightly larger number? Say 32 x 8?
One way would be to multiply 10 x 8 three times. But a faster way is to multiply 30 x 8, since 30 is a number ending in zero that is closest to 32. This gives us 30 x 8, which is the same as 3 x 8 with a zero at the end: 24, adding a zero at the end, gives us 240. We put this number “on hold” and then we multiply the 2 left over (remember, our original number was 32) times the 8: 2 x 8 = 16. We add this to the number “on hold”—240—giving us 240 + 16 = 256.
Check out the video for more SAT mental math practice!
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Chris Lele is the GRE and SAT Curriculum Manager (and vocabulary wizard) at Magoosh Online Test Prep. In his time at Magoosh, he has inspired countless students across the globe, turning what is otherwise a daunting experience into an opportunity for learning, growth, and fun. Some of his students have even gone on to get near perfect scores. Chris is also very popular on the internet. His GRE channel on YouTube has over 10 million views. You can read Chris's awesome blog posts on the Magoosh GRE blog and High School blog! You can follow him on Twitter and Facebook!
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