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Saturday, November 3, 2012

Multiplying Decimals

Now that we have finished up multiplication, and my students have a solid foundation of multi-digit multiplication, we have moved on to multiplying decimals (I had originally planned to go on to patterning and functions, but decided this was a more logical step in the sequence.  Patterning will be NEXT).  It always seems like such a daunting and scary task for students, but once students see how simple it is, it's just as "easy" as the multi-digit multiplication they already know how to do!

We started by creating a flow map to put into our math journals, which I made into an anchor chart to hang in the room...

I have seen this taught many different ways, but I felt like it was important for students to understand WHY they are placing the decimal where they are, rather than just counting up the decimal places and plopping it in.  That is why I have students do an estimate (once they get proficient, I allow them to do the estimates mentally and just put it up in the corner next to a problem they are doing so that they are sure they are close to the right answer.)  This helps students conceptually, especially when they get to problems like 1.2 x .34 .  If they estimate correctly (1 x 0 = 0), they should understand that their answer is going to be LESS THAN one.

I had a few kids get a little confused as to how you could get only a decimal answer if you are multiplying by a whole number.  We went back to the good 'ole trusty money comparison to do some investigating... If I had 4 dimes, 4 x .10, am I going to have a whole dollar? Nope, just .40 .

We did a lot of practicing in our math journals, and we explored place value in relation to multiplying decimals.  When I felt like the kids had a pretty good understanding of multiplying decimals, they began their assignment, which they are LOVING!  I basically created three different shops that they have to use their multiplying skills to shop at.  (I also created dividing decimals worksheets to go with them, too for when we begin that).   Pretty much anytime you include 1) shopping and 2) food in an assignment, the kids go crazy for it.  (We also did a shopping activity when we worked on adding and subtracting decimals, so I'll have to post about that sometime soon!)  I really tried to include problems that were not only real to life, but also had some depth and complexity to them, while allowing the kids to make some choices and take charge of the activity.

Multiplying (and Dividing) Decimals Activity Pack

You can see in the top right hand corner of most of this student's work that she has written the whole number estimate to check and see if her decimal placement is reasonable.

As they are finishing up, they have been working on these fun multiplying decimals task cards.  The set I made includes multiplying and dividing, so right now, I am having them determine if they are multiplication or division problems, then skipping the division ones to come back to later when I teach dividing decimals.  There is just something about task cards that kids can't get enough of. (Side Note: I am still busily working on finishing my task card board.  I have gotten so many comments from teachers who have taken a peek into my room and seen it!  It's so fun, and the kids are enjoying exploration all the cared, too.)

Multiplying Decimals Task Cards

Here is the FREE Decimals Quiz that I will be giving to my kids when we finish this unit!

Download your FREE Quiz or Review Here!

I also have a lot of other decimals products, and I recently combined ALL of my decimal products into a big bundle, which you can view here.  This pack has 3 sets of Decimals Task Cards (Adding & Subtracting Decimals, Multiplying & Dividing Decimals, and Decimal Concepts) and then my activity packets for adding and subtracting decimals as well as the activity pack I wrote about above for multiplying and dividing decimals.