Skill: Add 3-digit numbers without regrouping

To add 3-digit numbers without regrouping:

1. Line up the place values: hundreds under hundreds, tens under tens, and ones under ones.
2. Add the ones.
3. Add the tens.
4. Add the hundreds.
5. Each column’s sum must be $9$ or less, so no digit needs to be regrouped.

Example:

$$\begin{array}{r} 243\\ + 516\\ \hline \end{array}$$

First, add the ones:

$$3+6=9$$

Next, add the tens:

$$4+1=5$$

Then, add the hundreds:

$$2+5=7$$

Write each answer in its place:

$$\begin{array}{r} 243\\ + 516\\ \hline 759 \end{array}$$

So,

$$243+516=759.$$

This also matches expanded form:

$$243+516=(200+40+3)+(500+10+6)$$ $$=(200+500)+(40+10)+(3+6)=700+50+9=759.$$

Always add numbers with the same place value: ones with ones, tens with tens, and hundreds with hundreds.