Multiplying decimals · Usually met around P5
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, so that's three in all. Put the point back three places into 100 → 0.100 = 0.1. Not enough digits? Fill in zeros at the front: 0.2 × 0.4 → 2 × 4 = 8 → 0.08. Times a whole number works the same way: 0.3 × 4 → 3 × 4 = 12 → 1.2. Check: times a number bigger than 1, the answer is bigger than what you started with. Times a number less than 1, it's smaller. Column multiplication also places the point by counting decimal places.
Worked examples
Have a go first, then open each one for the answer and the working.
Example 1
0.9 × 5
Show the answer and working
Answer: 0.9 × 5 = 4.5
0.9 × 5: multiply as whole numbers, 9 × 5 = 45; 0.9 has one place, so put the point back one place into 45 → 4.5
Example 2
0.3 × 7
Show the answer and working
Answer: 0.3 × 7 = 2.1
0.3 × 7: multiply as whole numbers, 3 × 7 = 21; 0.3 has one place, so put the point back one place into 21 → 2.1
Example 3
9 × 0.2
Show the answer and working
Answer: 9 × 0.2 = 1.8
9 × 0.2: multiply as whole numbers, 9 × 2 = 18; 0.2 has one place, so put the point back one place into 18 → 1.8
Practise it in the game →Multiplying decimals · Free to start, unlimited questions, no download