Look at the sum on the board. Someone has stacked the two numbers using the right edge of the digits — the 7 sits directly above the 5:

   4.7
+ 0.85
  -----

Hands up: what is wrong with how these two numbers have been lined up?

Now watch four worked sums on the column-addition interactive. The decimal points line up in a single vertical line every time. Where one number we're adding has fewer places, a zero holds the empty column. Notice how the carry-1 jumps the same way it did with whole numbers.

Example 1: 4.7 + 0.85 = 5.55

The trailing-zero trick: 4.7 becomes 4.70 so the hundredths column has something to add.

Example 2: 0.3 + 0.27 = 0.57

Same idea on smaller numbers: 0.3 becomes 0.30.

Example 3: 1.05 + 2.345 = 3.395

The lonely thousandth: 1.05 becomes 1.050 so the thousandths column has a 0 above the 5.

Example 4: 9.99 + 0.01 = 10.00

The cascade regroup: every column tips over a 10, the carry-1 ripples all the way to the units, and the answer crosses one whole.

Today we work two sums together on the IWB.

• First: 6.4 + 2.85. Where does the decimal point go in the answer before any digit lands? Where does the trailing zero need to sit on 6.4?
• Then: 0.7 + 0.567. This one has two empty places to fill on 0.7, and the answer crosses one whole.