Mathnasium #MathTricks: Number Sense (Subtraction Part 2)

Here is a number sense trick to subtract without borrowing.

Let’s say we want to subtract 54 from 900. Since the digits in the ones and tens places of 900 are both less than the same digits in 54, we need to borrow to find the difference.

With this Mathnasium Math Trick, we avoid borrowing by moving the “difference window” to find an easier difference. We can change 900 to 899 by subtracting one, and account for the same change in 54. Subtracting one from 54, we have 53.

Now, the digits in the ones and tens places of 899 are larger than the same digits in 53. Using mental math, we find the difference of 846 and so:

900 – 54 = 846.

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