Stage 8 · Fractions & decimals
Multiplying decimals
Multiply as whole numbers, count the places.
How to do it
Method
Multiply as whole numbers, count the places
0.25 × 0.4: 25 × 4 = 100 → 0.100 = 0.1
Another way
Column multiplication
Columns: 25 × 4 = 100, 2 + 1 = 3 places → 0.1
In full
Multiply as if they were whole numbers, then count the decimal places: 0.25 × 0.4 → 25 × 4 = 100; 0.25 has two places and 0.4 has one, three in all, so put the point back three places into 100 → 0.100 = 0.1. Short of digits? Fill in zeros at the front: 0.2 × 0.4 → 2 × 4 = 8 → 0.08. Times a whole number works the same: 0.3 × 4 → 3 × 4 = 12 → 1.2. Check: times a number bigger than 1, the product is bigger than what you started with; times a number less than 1, it's smaller. Multiplying in columns also places the point by counting decimal places — the same answer.
Worked examples
Have a go first, then open each one for the answer and the working.
Example 1
0.2 × 9
Show the answer and working
Answer: 0.2 × 9 = 1.8
0.2 × 9: multiply as whole numbers, 2 × 9 = 18; 0.2 has one place, so put the point back one place into 18 → 1.8
Example 2
1.9 × 4
Show the answer and working
Answer: 1.9 × 4 = 7.6
1.9 × 4: multiply as whole numbers, 19 × 4 = 76; 1.9 has one place, so put the point back one place into 76 → 7.6
Example 3
0.8 × 5
Show the answer and working
Answer: 0.8 × 5 = 4
0.8 × 5: multiply as whole numbers, 8 × 5 = 40; 0.8 has one place, so put the point back one place into 40 → 4.0 = 4
Example 4
0.17 × 9
Show the answer and working
Answer: 0.17 × 9 = 1.53
0.17 × 9: multiply as whole numbers, 17 × 9 = 153; 0.17 has two places, so put the point back two places into 153 → 1.53
Practise multiplying decimals in the game →Stage 8 · Fractions & decimals · Free to start, unlimited questions, no download