Random, or Not?

Before you begin this activity, you might lke to read these definitions:

Add or Multiply Two Whole Numbers Together

Have you ever thought about what result you get:

In particular, are all last digits equally likely?

Example:

39 + 57 = 96 has last digit 6

38 × 45 = 1,710 has last digit 0.

So are the digits 0 to 9 all equally likely?

What's your guess?


Adding. Tick one of the following:

When you add two randomly selected whole numbersTick
Yes, last digits are all equally likely
No, last digits are not all equally likely


Multipling. Tick one of the following:

When you multiply two randomly selected whole numbersTick
Yes, last digits are all equally likely
No, last digits are not all equally likely

 

Let's see whether you guessed correctly ...

(Note: we give answers to the tables at the bottom of the page ... but only check them when you are done, or this wouldn't be an activity would it?)

Addition

Think about:

You will see that they all end in the digit 1.

So what do they have in common?

They are all sums of whole numbers whose last digits are 3 and 8 respectively. When we add a number ending in 3 to a number ending in 8, we always get a number ending in 1.

So all we need to consider are the last digits of the two numbers we are adding together.

We can do this by completing a table.

The following table is incomplete. Can you fill in the missing numbers?

Remember: just the last digit after addition, so with 6+7=13, we want the "3"

+0123456789
002358
12470
225701
3478 01
4468 03
5680 13
6680 23
780136
88035 6
901 368

You can now tally the numbers and complete a frequency table:

Last digitTallyFrequencyRelative
frequency
0tally 5tally 5100.1
1
2
3
4
5
6
7
8
9

 

Did you find all last digits are equally likely this time?

The answer is YES.
Each value 0 to 9 occurs exactly 10 times out of 100.
So they are all equally likely, just like when you throw a die.

Relative frequencies

Can you complete the last column of the table with the relative frequencies for each last digit?

Example:

0 occurs 10 times out of 100, so the relative frequency for 0 is 10/100 = 0.1

 

Multiplication

Think about:

You will see that they all end in the digit 8.

So what do they have in common?

They are all products of whole numbers whose last digits are 2 and 9 respectively. When we muliply a number ending in 2 with a number ending in 9, we always get a number ending in 8.

So all we need to consider are the last digits of the two numbers we are multiplying together.

The following table is incomplete. Can you fill in the missing numbers?

Remember: just the last digit after multiplication, so with 3×6=18, we want the "8".

×0123456789
00000000000
10134679
2046046
30392817
40820426
5050 0050 5
6028068
70718293
80640842
9096431

You can now tally the numbers and complete a frequency table:

Last digitTallyFrequencyRelative
frequency
0tally 5tally 5tally 5tally 5tally 5tally 2270.27
1
2
3
4
5
6
7
8
9

 

Did you find all last digits are equally likely this time?

The answer is still NO.
Last digit 0 occurs 27 times out of 100, but last digit 7 only occurs four times:

1 × 7, 3 × 9, 7 × 1 and 9 × 3

Relative frequencies

Can you complete the last column of the table with the relative frequencies for each last digit?

Example

0 occurs 27 times out of 100, so the relative frequency for 0 is 27/100 = 0.27

 

Conclusions

Did you predict the results correctly?


Addition gives equally likely results but multiplication does not ... how about that!

Could the relative frequences be useful somehow?

 

 

... Don't look past here until you have completed the activity! ...

 

Completed Tables

Here are the answers:

Addition

+0123456789
00123456789
1123456 789 0
223 456 78 901
3345 678 901 2
445 678 901 23
5567 890 123 4
667 890 123 45
7789 012 345 6
889 012 345 67
9901 234 567 8

Last digitFrequencyRelative
frequency
0100.1
1100.1
2100.1
3100.1
4100.1
5100.1
6100.1
7100.1
8100.1
9100.1
Total1001.0

 

Multiplication

×0123456789
00000000000
10123456789
20246802468
30369258147
40482604826
50505050505
60628406284
70741852963
80864208642
90987654321

Last digitFrequencyRelative
frequency
0270.27
140.04
2120.12
340.04
4120.12
590.09
6120.12
740.04
8120.12
940.04
Total1001.00