Multiplying decimals
Multiply without points, count them at the end
What is this about?
Something curious happens with multiplication: it's the only case where you do NOT need to line up the points. You can forget about them while multiplying and place the point at the end by counting how many there were.
The rule to remember
Multiply the two numbers as if they were whole numbers, ignoring the points. Then count how many decimals there are in total across both factors and place the point leaving that many digits on the right.
Example 1
2.4 × 0.3
- 1Drop the points and multiply as whole numbers: 24 × 3 = 72.
- 2Count the decimals: 2.4 has 1 and 0.3 has 1. That's 2 in total.
- 3In 72, count 2 digits from the right and place the point: 0.72.
Result: 0.72
Example 2
1.25 × 4
- 1Multiply without points: 125 × 4 = 500.
- 2Total decimals: 1.25 has 2 and 4 has none. Total: 2.
- 3Count 2 digits from the right in 500 and place the point: 5.00.
- 4Trailing zeros can be dropped: 5.
Result: 5
The most common mistake
Many people write
2.4 × 0.3 = 7.2
Why it's wrong
The multiplication was right (24 × 3 = 72) but only one decimal was counted instead of two. A quick check: 0.3 is less than 1, so the result must be SMALLER than 2.4. And 7.2 is bigger, so something's off.
Did it click?
The best way to find out is to try. These exercises are exactly on this topic.
Practise this topic