Decimal Addition

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Build the sum: set the ones, tenths and hundredths with the − / + buttons, then press Check

3D math game · Best for grades 4-5

Decimal Addition

Decimal Addition has students build the sum of two decimal numbers column by column using digit wheels for ones, tenths, and hundredths. The game makes the align-then-add procedure concrete: each column is set independently so carrying across the decimal point becomes visible.

What students practice

Decimal Addition practises the column-by-column algorithm with decimal numbers: aligning the decimal point, adding like places (hundredths with hundredths, tenths with tenths), and carrying across columns. These are the same skills as whole-number addition, extended to fractional positions.

  1. Read the two decimal addends shown on screen.
  2. Add the hundredths digits first; use the hundredths wheel to set that digit of the sum.
  3. If the hundredths total is 10 or more, carry 1 to the tenths column.
  4. Repeat for tenths, then ones, carrying as needed.
  5. Press Check when all three wheels show the correct sum.

How to use it at home or in class

For 1.47 + 0.86: hundredths 7+6=13, set 3 and carry 1; tenths 4+8+1=13, set 3 and carry 1; ones 1+0+1=2. Answer 2.33. Each step is done on a separate wheel, so no digit is lost.

The most common error is ignoring column alignment and adding 1.47 + 0.86 as if they were 147 + 86. Separate wheels for each place prevent that by forcing students to add only the matching column at a time.

Use it for individual practice, paired work, or a short class demonstration before worksheets.

Start with the game above, then continue with written practice to reinforce the skill.

Common questions

How does the column-wheel interface teach decimal-point alignment?
Each wheel is labelled ones, tenths, or hundredths and only accepts its own digit, so students cannot add a tenths digit into the ones column by accident. The structure enforces alignment without a written grid.
Which grades is Decimal Addition for?
Grades 4-5, where students extend column addition to decimals with tenths and hundredths.
Does the game cover subtraction of decimals too?
No. Decimal Addition focuses on the addition algorithm with carrying. Decimal subtraction with borrowing is a separate skill practised in a different activity.