Three-Addend Sums

Three-Addend Sums

When you add three whole numbers, the order can make the calculation easier. The total stays the same no matter how you group the numbers, so choose the grouping that is simplest for you.

1) Look for an easy pair

Check whether two of the numbers make a friendly sum, such as 10, 20, 100, or another round number. If so, add those two first.

2) Add in the easiest order

You can compute the sum by grouping the first two numbers, then adding the third, or by grouping the last two first. For example:

If 8 + 2 + 5, first do 8 + 2 = 10, then 10 + 5 = 15.

• If 6 + 4 + 9, first do 6 + 4 = 10, then 10 + 9 = 19.

3) Check your answer

After finding the total, estimate whether it makes sense. A quick check is to see if your result is larger than each addend and close to the size you expect. You can also add the numbers again in a different order to confirm the same total.

Helpful habit

If one number is small, it is often useful to combine it with a number that helps make a round sum. This reduces mental work and helps avoid mistakes.