Here is a sum to look at: 3.4 + 0.275. Quick question for you: if we just lined up the very last digits of each number and added straight down, what would go wrong? Is it right to put the 4 above the 5?

The interactive works through four decimal sums, lining up the decimal points and filling in the empty columns before adding or subtracting. Watch how each number is padded with zeros so every column has a digit, and keep an eye on the carries and borrows.

Let's work this one through together on the board: 5.6 + 2.34. We line up the points first, fill the gap with a zero so 5.6 becomes 5.60, then add one column at a time from the right.

Today we tackle these subtractions together at the board: 7.2 − 3.485, then 15.0 − 6.074, then a thinking one — find a decimal that subtracts from 9.5 to leave exactly 3.275.

What we learned today

• Line up the decimal points before you add or subtract decimals.
• Fill the empty columns with zeros so every column has a digit.
• Carry and borrow across the decimal places exactly as you do across whole numbers.

Coming up

Next we move on to multiplying multi-digit whole numbers, breaking each factor into place-value parts and using the area model to see every partial product.