Skill: Add decimals

To add decimals, line up digits with the same place value. The decimal points should be directly above each other.

Example:

$$5.68+2.459$$

First, write the numbers with their decimal points aligned. Add a zero to make the first number have thousandths:

  5.680
+ 2.459

Now add from right to left:

1. Thousandths: $0+9=9$
2. Hundredths: $8+5=13$. Write $3$ and regroup $1$ to the tenths place.
3. Tenths: $6+4+1=11$. Write $1$ and regroup $1$ to the ones place.
4. Ones: $5+2+1=8$.

The decimal point stays aligned:

  5.680
+ 2.459
-------
  8.139

Therefore,

$$5.68+2.459=\boxed{8.139}$$

The added zero does not change the value of $5.68$ because $5.68=5.680$. You can estimate to check: $5.68$ is about $5.7$, and $2.459$ is about $2.5$, so the sum should be about $8.2$. The answer $8.139$ is reasonable.