Math CentralQuandaries & Queries


Question from telly, a student:

how to do this sum 0.24*0.398

We have two responses for you


Think of 0.24 as 24/100 and 0.398 as 398/1000 then


Thus you can multiply 0.24 and 0.398 my multiplying 24 and 398 and then dividing by 100,000. Thus calculate 24 × 398 and then move the decimal place 5 places to the left.




Use exactly the same method you would use to find

24 * 398

but adjust the place value. Three ways to do this:

  1. Work out the place value for each partial product. Thus

    [.008*0.24 =] 0.00192
    Your teacher may prefer this method if you have to show your work in detail

  2. You can actually find 24 * 398 and fix place value after the calculation is completed. To get from 0.24 to 24 you have to shift the place value 2 places. To get from 0.398 to 398 you have to shift it 3 places. So to get the final answer you have to shift back 5 places.

  3. 0.24 is about a quarter and 0.398 is about 0.4. Use this to estimate the answer, find 24 * 398, and shift the place value till your answer is near your estimate.

Good Hunting!

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