Two-Digit Mental Subtraction

When subtracting two-digit numbers mentally, our aim is to simplify the problem. Let’s begin with this simple subtraction problem :

        -24     (20 + 4)

Our first step is to subtract 20 from 87 (87 – 20 = 67). Then we subtract 4 to reach the simpler subtraction problem 67 – 4 for our final answer of 63.

The above example is easy because there is no borrowing involve. If a two digit subtraction problem would required borrowing, then round the second number up to a multiple of ten. Subtract the rounded number, then add back the difference. For example :

        -29     (30 – 1)

So our steps are round 29 to 30, compute 84 – 30 (which is equal to 54), then add back 1 to get 55 as your final answer.

With just a little bit of practice, you will become comfortable working subtraction problems mentally 🙂 .



Posted on August 15, 2009, in Info Matematik. Bookmark the permalink. 3 Comments.

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