Here is a sum written out for adding: 4.7 + 0.85. But look closely at the way it is set up. Is this layout right? What looks wrong about how the two numbers are lined up?

The interactive adds three decimal sums in columns, one at a time. Watch how the decimal points line up, and notice when an empty place needs a zero to fill it.

Today we work through these sums together: 1.05 + 2.34, then 0.6 + 0.27, then 4.8 + 3.65. For each one we will line up the decimal points first, add a trailing zero where a column is empty, then work each column from the right.

Now we work through a short ladder of decimal sums together, each one a little harder than the last. For every sum we line up the points, fill any empty column with a zero, then add. We check each answer together before the next sum appears.

What we learned today

• Line up the decimal points one under the other, not the right edge of the digits.
• Fill any empty decimal place with a trailing zero so every column has a digit.
• Add each column from the right, carrying a ten left just like with whole numbers.

Coming up

Next we sharpen our subtraction strategies — counting on, taking away, and breaking a number apart — so we can pick the quickest way for each subtraction.