Multiplying two decimals mentally
Multiplying two simple decimals can be done mentally using knowledge of place value and times tables. It is very similar to multiplying a decimal by an integer (whole number), but the place value of both numbers changes.
The only difference between the calculation 0.4 x 0.2 and 4 x 2 is the place value of the two numbers. 0.4 is 10 times smaller than 4 and 0.2 is 10 times smaller than 2. So the answer to 0.4 x 0.2 will be 100 times smaller, because there are two lots of division by 10, and 10 x 10 = 100. If 4 x 2 = 8, then 0.4 x 0.2 = 0.08. One helpful little trick is considering the number of decimal places in your question and answer. If we look at 0.4 x 0.2, there are two numbers after decimal points in the question. Therefore, there must be two numbers after the decimal point in the answer, so the answer is 0.08. 
First Steps 
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Next StepsMultiplying two decimals (written)

External links
 IXL: Online questions with solutions.
 Mymaths: Interactive lesson on multiplying decimals. Note that this also covers questions on multiplying decimals using written methods too. Test yourself with the online homework.