Mathnasium #MathTricks: Number Sense (Subtraction Part 2)

Dec 8, 2021

Here is a number sense trick to subtract without borrowing.

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

With this Mathnasium #MathTrick, we can avoid borrowing by moving the “difference window” to find an easier difference. We can change 900 to 899 by subtracting 1 and accounting for the same change in 54. Subtracting 1 from 54, we have 53.

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

900 – 54 = 846.

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