Strategies for Multiplying and Dividing Decimals

May 11, 2011 | Posted in: Class Updates

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Yesterday, we started working on multiplying decimals.  To all the parents who saw the homework and thought that it was downright loonie that we were using a three step process rather than just multiplying the decimals outright, don’t worry – we will get there.  Using the three step process teaches valuable number sense skills that help kids to understand the nature of numbers and why certain multiplication tasks result in certain answers.

Today, we started working on dividing decimals.  The bad news:  Again, we used three wacky steps, rather than just learning how to divide decimals outright.  The good news:  The steps are the same.  In case you need a refresher (kids), here are the steps:

1. Estimate the answer (by rounding the decimal up or down).
2. Multiply or divide as if there was no decimal point.  (Just take it out.)
3. Consolodate your answers.  Use the ballpark estimate from step 1 to figure out what your answer should be close to, and use the digits from step 2 to figure out what numerals should make up your answer.  Insert the decimal point appropriately.

Here are some examples:

Multiplication:

3.28 x 5 = ?

Step 1:  Round 3.28 down to 3.  Multiply 3 x 5.  The answer, 15, if your ballpark estimate.  Your answer should be close (two digits).

Step 2:  Remove the decimal from 3.28.  Multiply 328 (yes, three hundred twenty eight) by 5.  328 x 5 = 1,640.

Step 3:  Using the digits 1  6  4  0, add a decimal that gives you an answer close to your ballpark estimate of 15.  In this case, put a decimal between 6 and 4.  Your answer is 16.40.

 

Division:

36.79/3=?

Step 1:  Again, round your decimal.  36.79 rounds up to 37.  When you divide with long division, you get an answer of 12 r1 (remainder of 1).  This tells you that your answer aught to be close to 12, certainly in the same place value.

Step 2:  Use long division to divide without a decimal point.  In other words: 3679/3=?.  You get the answer of 1,126r1.

Step 3: You need to use the digits 1  1  2  6 to find an answer close to your ballpark estimate of 12.  Insert the decimal point to make 12.26.