Mental Math Strategies

Addition by equivalent sums

Practice Table of Contents

Introduction

Some addition problems are easier than others. The sum of 20 and 32, for example, is easy because it is straightforward: add the tens (2 + 3), and then the ones (0 + 2), to arrive at 52.

Adding 18 and 34, however, results in too many ones to use the same strategy. We can make this sum easier by moving things around a bit.


Step by step | Addition by equivalent sums

  1. Identify the number that is closer to a ten.
  2. Add or subtract to make this number into the ten.
  3. Add or subtract from the other number to maintain the same total value.
  4. Add the new numbers to find an equivalent sum.

Example | Addition by equivalent sums

Q: 18 + 34

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A: The sum of 18 and 34 would be easier if we add 2 to 18 to make 20. Since we add 2 to 18, we must subtract 2 from 34 to maintain same total value:
18 + 34 = 20 + 32 = 52.

Practice | Addition by equivalent sums