multiplying-decimals-by-decimals-the-shortcut

The shortcut for multiplying decimals by decimals is this: you multiply as if there were no decimal points, and then you place a decimal point in the answer in such a manner that the number of decimal digits in the answer is the SUM of the decimal digits in all the factors.

We practice using this rule with several problems, and then I show where the rule comes from, using fraction multiplication.

Consider this example: 2/10 x 6/100 = 12/1000. The number of zeros in the denominator (in the power of ten) corresponds to the number of decimal digits:

2/10 is 0.2 (one decimal digit)

6/100 is 0.06 (two decimal digits)

The answer, 12/1000, is 0.012 (three decimal digits).

As we multiply powers of ten (the denominators), we simply ADD the number of zeros to get the answer. And that is why we ADD the decimal digits to get the amount of decimal digits for the answer.

This lesson is meant for 5th grade math (and onward).

Check out also the 1st part of this lesson, where we explore SCALING, at https://www.youtube.com/watch?v=uqCEojNpjUg

Tagged under: multiply decimals,multiply decimal decimal,decimal multiplication,rule,shortcut,decimal digits,5th grade,math

Clip makes it super easy to turn any public video into a formative assessment activity in your classroom.

Add multiple choice quizzes, questions and browse hundreds of approved, video lesson ideas for Clip

Make YouTube one of your teaching aids - Works perfectly with lesson micro-teaching plans

1. Students enter a simple code

2. You play the video

3. The students comment

4. You review and reflect

* Whiteboard required for teacher-paced activities

With four apps, each designed around existing classroom activities, Spiral gives you the power to do formative assessment with anything you teach.

Quickfire

Carry out a quickfire formative assessment to see what the whole class is thinking

Discuss

Create interactive presentations to spark creativity in class

Team Up

Student teams can create and share collaborative presentations from linked devices

Clip

Turn any public video into a live chat with questions and quizzes

Tried out the canvas response option on @SpiralEducation & it's so awesome! Add text or drawings AND annotate an image! #R10tech

Using @SpiralEducation in class for math review. Student approved! Thumbs up! Thanks.

Absolutely amazing collaboration from year 10 today. 100% engagement and constant smiles from all #lovetsla #spiral

Students show better Interpersonal Writing skills than Speaking via @SpiralEducation Great #data #langchat folks!

Dr Ayla Göl
@iladylayla

A good tool for supporting active #learning.

The Team Up app is unlike anything I have ever seen. You left NOTHING out! So impressed!