Easy Math Tricks: Multiplying Large Numbers by 5

In grade school children learn their multiplication tables. For the rest of their lives people usually remember these multiplication tables but when they get to multiplying large numbers they can't do it without a calculator because they were never taught how to multiply by larger than 12. This is an easy trick for learning how to multiply large numbers by five.

Five is one of the easiest numbers to learn in the multiplication table. If you ask somebody what 5 times 7, 5 times 5 or 5 times 9 is then they can usually rattle off the answers. But how quickly can you figure out 5 times 32? How about 5 times 126? 5 times 481? 5 times 2843? 5 times 482462?

Multiplying these numbers by five is a lot easier than it looks. This trick is slightly different for even numbers and odd numbers. Even numbers are always easier to deal with so let's start with even numbers.

Multiplying large even numbers by 5

Step One: If you are multiplying a large even number by five then the first step is to divide the large number by 2, or to cut it in half.

Step Two: After you have divided the number in two, or cut the number in half, add a zero to the end.

That's it. It's really that easy. In order to show that this rule really does work let's use an example that you'll know the answer to.

5 times 10

Step One: Divide by 2

10/2 = 5

Step Two: Add zero to the end of the number.

50

5 times 10 does equal 50, correct? Let's try another one you'll know.

5 times 100

Step One: Divide by 2

100/2 = 50

Step Two: Add zero to the end of the number

500

Now let's try some more difficult problems that you probably don't know off the top of your head.

5 times 32

Step One: Divide by 2

32/2 = 16

Step Two: Add zero to the end of the number

160

5 times 32 = 160

5 times 126

Step One: Divide by 2

126/2 = 63

Step Two: Add zero to the end of the number

630

5 times 126 = 630

5 times 482462

Step One: Divide by 2

482462/2 = 241231

Step Two: Add Zero to the end of the number

2412310

5 times 482462 = 2412310

This trick is just slightly different with an odd number.

Multiplying large odd numbers by 5

Step One: If you are multiplying a large odd number by 5 then your first step is to divide by 2 or cut the number in half.

Step Two: If you divide an odd number by two then you're going to be left with a .5 remainder. Move the decimal point to the right one spot, making your number end with 5 instead of .5.

Again let's try an example that you'll know the answer to.

5 times 9

Step One: Divide by 2

9/2 = 4.5

Step Two: Move the decimal to the right.

45

5 times 9 = 45.

Let's try some harder numbers.

5 times 481

Step One: Divide by 2

481/2 = 240.5

Step Two: Move the decimal to the right.

2405

5 times 481 = 2405

5 times 2843

Step One: Divide by 2

2843/2 = 1421.5

Step Two: Move the decimal to the right

14215

5 times 2843 = 14215