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Lesson 3

MULTIPLY BY POWERS OF 10

DIVIDE BY POWERS OF 10

The Meaning of Percent


In this Lesson we will learn to multiply and divide by a power of 10 -- simply by moving the decimal point. Having done that, we can then understand scientific notation.

These are problems that do not require a calculator, and certainly should not be calculated in writing.


In this Lesson, we will answer the following:

  1. How do we multiply a whole number by a power of 10?
  2. How do we multiply a decimal by a power of 10?
  3. How do we divide a decimal by a power of 10?
  4. What do we mean by the number of decimal places?
  5. How do we divide a whole number by a power of 10?

    Section 2: The Meaning of Percent

  6. What does "percent" mean?
  7. How can we take 1% of a number?
  8. How can we take 10% of a number?
  9. How do we change a percent to a number?
  10. How do we change a number to a percent?

    Section 3:

  11. When is a number written in "scientific notation"?
  12. How do we convert units in the metric system?


 1.   How do we multiply a whole number by a power of 10?
36 × 10
 
  Add on as many 0's as appear in the power.

  Examples.    36 × 10  =  360     Add on one 0.
  36 × 100  =  3600     Add on two 0's.
  36 × 1000  =  36,000     Add on three 0's.


 2.   How do we multiply a decimal by a power of 10?
 
7.32 × 10
 
  Move the decimal point right as many places
as there are zeros in the power.

Examples.   

7.32 × 10 = 73.2 Move the decimal point one place right.
7.32 × 100 = 732 Move the point two places right:  732.  However, since all the digits fall to the left of the decimal point, the answer is a whole number, 732, which we write without a decimal point.
7.32 × 1000 = 7,320  Move the point three places right.  To do this, we must add on a 0.
Again, the answer is a whole number.

Problem.   If 5 pounds of sugar cost $2.79, how much will 50 pounds cost?

Answer.  Since 50 pounds are ten times 5 pounds, they will cost ten times more.  Move the decimal point one place right:  $27.90.  Since money has two decimal places, we added on a 0. (Lesson 3, Question 8)
 3.   How do we divide a decimal by a power of 10?
 
63.4 ÷ 10
 
  Move the decimal point left as many places as there are 0's in the power.  If there are not enough digits, add on 0's.

Examples.    63.4 ÷ 10 = 6.34 Move the point one place left.
  63.4 ÷ 100 = .634   Move the point two places left.
  63.4 ÷ 1000 = .0634    Move the point three places left.  To
do this, add on a 0.

Finally, we must see how to divide a whole number by a power of 10.  In Lesson 1 we saw that when a whole number ends in 0's, we simply take off 0's. (Lesson 1, Question 11)

265,000 ÷ 100 = 2,650

But when a whole number does not end in 0's -- as in 265 -- then there are no 0's to chop off!  We will see that we must separate digits from the right.  First:
 4.   What do we mean by the number of decimal places?
 
621.5
 
  They are the number of digits to the right of the decimal point.

  Examples.    621.5  has one decimal place.
 
   6.215  has three decimal places.
 
   6,215  has no decimal places.  It is a whole number.



 5.   How do we divide a whole number by a power of 10?
 
265 ÷ 10
 
  Starting from the right of the whole number, separate as many decimal places as there are 0's
in the power. If there are not enough digits,
add on 0's.

  Examples.   265 ÷ 10   =  26.5 Starting from the right of 265,
separate one decimal place.
 
   265 ÷ 100   =  2.65 Separate two decimal places.
 
   265 ÷ 1000   =  .265 Separate three decimal places.

Again, as in Lesson 1, consider this array:

Place values

As we move down the list -- as we push the digits one place left -- the number has been multiplied by 10, because the next place is worth 10 times more.  (As we move from 2.658 to 26.58, we go from 2 ones to 2 tens.)  It appears, though, as if the decimal point has shifted one place right, or, with whole numbers, that a 0 has been added on.

As we move up the list -- as we push the digits to the right -- each number has been divided by 10.

And so we can easily multiply or divide by a power of 10 because of the written system itself.  Each place belongs to the next power of 10.


At this point, please "turn" the page and do some Problems.

or

Continue on to the Section 2:  The meaning of percent.


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