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Addition, subtraction, and estimation: FAQ

Frequently asked questions about addition, subtraction, and estimation.

What is the difference between rounding and estimating?

Rounding is one way of estimating. When we round, we adjust a number to the nearest "round" number (like the nearest ten, hundred, or thousand). Estimating can also involve guessing or using other strategies to come up with an approximate answer.
Here are a couple of the exercises that practice rounding:

What is multi-digit addition?

Multi-digit addition is just like regular addition, except that the numbers we're adding have more than one digit. The key is to line the numbers up so that the place values match. For example, to add 265 and 348, we would write:
265+348\begin{aligned} &265\\ +&348\\ \hline \end{aligned}
Starting on the right side, we add 5 (from the ones place) and 8 (from the ones place) to get 13. We write the 3 down and carry the 1 (from the tens place) over to the next column.
Next, we add 6 (from the tens place) and 4 (from the tens place) to get 10. We add the 1 that we carried over to get 11. We write the 1 down and carry the 1 (from the hundreds place) over to the next column.
Finally, we add 2 (from the hundreds place) and 3 (from the hundreds place) to get 5. We add the 1 that we carried over to get 6.
So the sum of 265 and 348 is 613.
Practice with our exercise Multi-digit addition.

What is multi-digit subtraction?

Multi-digit subtraction is similar to multi-digit addition, except that we're taking one number away from another. Again, the key is to line the numbers up so that the place values match. For example, to subtract 278 from 437, we would write:
437278\begin{aligned} &437\\ -&278\\ \hline \end{aligned}
Starting on the right side, we subtract 8 (from the ones place) from 7 (from the ones place). Since we don't have enough ones in the ones place, we need to regroup from the tens place. We can take one ten from the tens place, which leaves us with 2 in the tens place. We add that ten to the 7 in the ones place, which gives us 17. Now we can subtract 8 from 17 to get 9.
Next, we subtract 7 (from the tens place) from 2 (from the tens place). Again, we need to regroup from the next place value. We take one hundred from the hundreds place, which leaves us with 3 in the hundreds place. We add that hundred to the 2 in the tens place, which gives us 120. Now we can subtract 70 from 120 to get 50.
Finally, we subtract 2 (from the hundreds place) from 3 (from the hundreds place) to get 1.
So the difference between 437 and 278 is 159
Practice with our exercise Multi-digit subtraction.

When do we use these skills in the real world?

We use these skills all the time! We might round numbers when we're estimating the cost of items at the store, or when we're trying to get a rough idea of how much time a project will take. We use multi-digit addition and subtraction when we're working with larger numbers, like when we're balancing our bank account or measuring distances.

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