Mental Math Trick - Easy way to subtract!

The last couple of weeks, we've been reviewing divisibilty rules.  This week, we're starting off the 2022 new year with a cool mental math trick to help with subtracting across multiple zero's!

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Struggle trying to subtract when you have to borrow across multiple 0's?  This week's tip is a way to do subtraction without having to borrow!

In our example, we are subtracting 54 from 900.  Usually when we're subtracting using pencil and paper, we would have to borrow, since the values of the digits in the tens and ones places in 900 are both less than those in 54.  

This Mathnasium #MathTrick moves what we call the “difference window” (the distance between 2 numbers) to find an easier difference that avoids borrowing. We change 900 to 899 by subtracting 1 and then do the same change in 54 by subtracting 1 to get 53.  SInce we subtracted 1 from both numbers the "difference window" remains the same.

Now, the values of the digits in the ones and tens places of 899 are both larger than the those of 53. Using mental math, the difference of 899 – 53 is 846, and so

900 – 54 = 846.

Try it yourself with our practice sheet below!