Multiplication can Increase or Decrease a Number

Question: Does multiplication always increase a number?

Further Explanation

So, multiplying can have a reducing effect when multiplying a positive number by a fraction which is less than one. But this can still be confusing. While we accept the above, the concept of ' a number times 8' continues to be perceived as an increase. How then can we attach a meaning to so that this will be perceived as decreasing?

When multiplying by a whole positive number, e.g. 6 times 5, we understand this as being 5 added over and over again, how ever many times – six times in this example. But this interpretation of times does not quite work with fractions. If we ask how many times , the answer is "not quite once".

Again we need to put the term multiplying into a context with which we can identify, and which will then make the situation meaningful.

We want to buy 30 roses which are sold in bunches of 5, so we ask for "6 of the 5-rose bunches". In this way, the word times also often means of. If we try using the word of when times appears to have an unclear meaning, we get of 8 rather than times 8. Indeed we know what of 8 means – namely 4.

So, by using of instead of times we are able to understand the concept of multiplying by a fraction and how this can have a reducing effect when the fraction is smaller than 1.

This also helps us to understand how we multiply by a fraction, and why the method works:

the 4 which results from (or of 8) can be reached by dividing 8 by 2;

similarly, the 5 which results from (or of 15), (or a third of fifteen) can be reached by dividing 15 by 3.

Generalising this result gives:

Negative numbers

When your bank balance is +4 pounds you have £4.

When your bank balance is –4 pounds you owe £4.

Owing is the opposite of having , so we find that we can associate the concept of 'minus' with '(the) opposite (of)'. This also works in reverse.

Thus, ( -4) × 8 means " owing £4, eight times over" or "owing £32" which is - £32 .

Now - 32 is smaller than 8, so we have illustrated another case where multiplying has a reducing effect, i.e. when multiplying by a negative number.

Note that, using the method shown above, it follows that -1 × 8 = - 8 , and vice versa.

Exercise

Are the following statements:

• sometimes false
• always true and always false
• sometimes true

1. Multiplication of a positive number by a number greater than 1 always increases the number.
2. Multiplication of a positive number by a positive number between 0 and 1 always increases the number.
3. Multiplication of a negative number by a positive number always increases the first number.

Solution

1. Always true
2. Always false, as multiplication of a positive number by a number between 0 and 1 will always reduce the number. (e.g. etc.)
3. Sometimes false and sometimes true; e.g. for the number – 8, 2 × (-8) = -16, so the number is decreased, whereas the number increases in the example below: