Skill: Add multi-digit whole numbers

To add multi-digit whole numbers:

1. Line up the numbers by place value: ones under ones, tens under tens, hundreds under hundreds, and so on.
2. Add from right to left, starting with the ones.
3. If a column adds to $10$ or more, regroup: write the ones digit in that column and carry the tens digit to the next column.

Example:

$$\begin{array}{r} 3,758,496\\ +2,684,759\\ \hline \end{array}$$

Add each place-value column from right to left:

• Ones: $6+9=15$. Write $5$ and carry $1$ ten.
• Tens: $9+5+1=15$. Write $5$ and carry $1$ hundred.
• Hundreds: $4+7+1=12$. Write $2$ and carry $1$ thousand.
• Thousands: $8+4+1=13$. Write $3$ and carry $1$ ten thousand.
• Ten-thousands: $5+8+1=14$. Write $4$ and carry $1$ hundred thousand.
• Hundred-thousands: $7+6+1=14$. Write $4$ and carry $1$ million.
• Millions: $3+2+1=6$.

So,

$$\begin{array}{r} 3,758,496\\ +2,684,759\\ \hline 6,443,255 \end{array}$$

The sum is $\boxed{6,443,255}$.

You can check that the answer is reasonable by estimating:

$$3,758,496\approx3,800,000 \qquad\text{and}\qquad 2,684,759\approx2,700,000$$

Since $3,800,000+2,700,000=6,500,000$, the exact answer $6,443,255$ is reasonable.