Multiplying decimals

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.

Example 1

2.4 × 0.3

1. 1Drop the points and multiply as whole numbers: 24 × 3 = 72.
2. 2Count the decimals: 2.4 has 1 and 0.3 has 1. That's 2 in total.
3. 3In 72, count 2 digits from the right and place the point: 0.72.

Result: 0.72

Example 2

1.25 × 4

1. 1Multiply without points: 125 × 4 = 500.
2. 2Total decimals: 1.25 has 2 and 4 has none. Total: 2.
3. 3Count 2 digits from the right in 500 and place the point: 5.00.
4. 4Trailing zeros can be dropped: 5.

Result: 5

Practice exercises

Solve them in your notebook, then check the solution.

1. 1. What is 1.4 × 8?

   Show the solution

   11.2

   Here you DON'T need to line up the decimal points. Multiply as if they weren't there and place the point at the end.

2. 2. What is 4 × 9?

   Show the solution

   36

   Here you DON'T need to line up the decimal points. Multiply as if they weren't there and place the point at the end.

3. 3. What is 8.3 × 4?

   Show the solution

   33.2

   Here you DON'T need to line up the decimal points. Multiply as if they weren't there and place the point at the end.

4. 4. What is 4.6 × 8?

   Show the solution

   36.8

   Here you DON'T need to line up the decimal points. Multiply as if they weren't there and place the point at the end.

5. 5. What is 5.4 × 5?

   Show the solution

   27

   Here you DON'T need to line up the decimal points. Multiply as if they weren't there and place the point at the end.