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Lesson 2: Multiplying decimals strategies

# Estimating decimal multiplication

This video is all about using estimation to simplify the process of multiplying decimals. It emphasizes the idea that rounding numbers to their nearest whole can make multiplication easier and quicker, especially when you're trying to do it in your head!

## Want to join the conversation?

• Hi this video helped me enough but I am still confused how to do your estimate problems can you tell me some more information?
• You round your numbers to the nearest tenths, hundredths,and ones, then you do your old buddy thing: Multiply
• whats 450x24 and how do you do it?
• 450*24=10800
I got this answer by using the normal algorithm method.

Step 1: You jot down the numerals like so:
450
24
----
----
(You might wonder what the dotted lines under the equation are for, it's for writing down the answer)

Step 2: You multiply the units digits in both numbers like so:
0*4=0

(So after step 2 the equation would be like:
450
24
----
0
----

Step 3: You multiply the tens digit of the first numeral with the units digit of the second numeral.

Step 4: I guess you've caught the sequence by now. All you have to do this time is multiply the hundreds digit of the first numeral by the units digit of the second numeral.

Your equation would look like this so far:

450
24
----
1800
----

You might be wondering about the tens digit in the second numeral because we haven't even used it once yet right?

Don't worry I'll teach you in the next few steps:

(Hot Tip: Write down a plus sign underneath the units digit of your new answer so far)

Step 5: Now we come back to the units digit of the first numeral and multiply that by the tens digit of the first numeral.

Step 6: And the pattern starts all along again, but this time with the tens digit of the second numeral. This time you multiply the tens digit of the first numeral by the tens digit of the second numeral.

Step 7: Again, you multiply the hundreds digit of the first numeral by the tens digit of the second numeral.

Your equation will look like this so far:

450
24
----
1800
900+
----

Step 8: It's pretty obvious by now that all we have to do now is add the new answers we have acquired.

The new equation will look like this:

450
24
----
1800
900+
------
10800
------

Hope this helps :)
• I don't understand this whole video. Can someone help me?
• ok what he is doing is estimating so for EX 199 x 7.8 it is approximately equal to 1600 because 199 is equal to 200 and 7.8 is equal to 8.0 or just 8 because there's a zero next to it not a 1,2,3,4or any other number. Then multiply the 8 and the 200 you would get 1600
• Hi Sal, is there a specific strategy that will help solve multiplication with decimals for the exact sum?
• There is a few strategies. One is to use long multiplication, and just make sure that you have the correct amount of decimals. Example would be 0.25 x 1.23, you would first do 3 times 25, gives 75, then 20 times 25 for 500. and then 100 times 25 for 2500. Then add up for 3075, then you say first number had 2 decimal spaces, second also had 2, 2 plus 2 is 4, so you have 4 decimal places and get 0.3075.
Of course, this isn't the only way, you could also turn one or both decimals into fractions. For the example you could say 123/100 * 1/4 then multiply fractions to get 123/400. If you wanted to then turn that fraction into a decimal to get your answer you once again have different options but I would do long division.
• I got the first two of the questions correct, but couldn't complete the last question, which involved two decimals. I am trying to get an exact answer. Could someone help me out please?
• Hi there, have you tried rounding? I’ll provide an example. 12.6 x 5.2, in this case you round to get the problem 13 x 5. You are given 4 choices, (most of the time they are not exact) 70, 300, 1020 and 2340. To fine the answer you round your decimals to create a whole number and then multiply. In this case, 13 x 5 = 65, now you have to pick which one of your options is correct. We know that 70 is the correct option because it is the closest to 65.
(1 vote)
• I just cannot get this lol
• When we do decimal multiplication, it might be difficult for us to compute by hand (or mentally).

Therefore, we usually round the decimal to the nearest integer.

3.14 ≈ 3

For others, it depends on how many digits it have.

But to be honest, I think this really isn't that necessary to learn, since estimation depends on situation, and should be based on intuition and not under a set of procedures with definite answer.