Adding and subtracting decimals
It all comes down to lining up the point
What is this about?
A decimal number has a whole part and a part smaller than 1, separated by a point. In 3.45 you have 3 units, 4 tenths and 5 hundredths. You see it every day in prices: $1.95 is one dollar and ninety-five cents.
The rule to remember
Write the numbers one under the other with the point right above the point. If one is missing decimals, add zeros on the right to match. Then add or subtract as usual and bring the point straight down.
Example 1
12.4 + 3.75
- 1One has one decimal and the other has two, so match them: 12.4 becomes 12.40.
- 2Stack them with the points lined up: 12.40 above 3.75.
- 3Add normally from right to left: 0 + 5 = 5, 4 + 7 = 11 (carry 1), 2 + 3 + 1 = 6, bring down the 1.
- 4Bring the point straight down.
Result: 16.15
Example 2
8 - 2.6
- 18 has no decimals, but you can write it as 8.0 without changing its value.
- 2Line up the points: 8.0 above 2.6.
- 3Subtract: since 0 is smaller than 6, you borrow. 10 - 6 = 4, leaving 7 above.
- 47 - 2 = 5, and bring the point down.
Result: 5.4
The most common mistake
Many people write
12.4 + 3.75 = 15.79
Why it's wrong
The numbers were lined up on the right instead of by the point, adding tenths to hundredths. Adding the zero (12.40) prevents this mistake almost every time.
Did it click?
The best way to find out is to try. These exercises are exactly on this topic.
Practise this topic