## The Units Digits of Big Powers

Try this one:

What is the units digit of ?

A) 1

B) 3

C) 5

D) 7

E) 9

Let’s begin by looking for a pattern as we increase the exponent.

= 13 (units digit is 3)

= 169 (units digit is 9)

= 2197 (units digit is 7)

Aside: As you can see, the powers increase quickly! So, it’s helpful to observe that we need only consider the units digit when evaluating large powers. For example, the units digit of is the same as the units digit of , the units digit of is the same as the units digit of , and so on.

Continuing along, we get:

has units digit 3

has units digit 9

has units digit 7

has units digit **1**

has units digit 3

has units digit 9

has units digit 7

has units digit **1**

Notice that a nice pattern emerges. We get: 3-9-7-**1**-3-9-7-**1**-3-9-7-**1**-…

As you can see, the pattern repeats itself every 4 powers. I like to say that the “**cycle**” equals **4**

Now that we know the cycle is 4, we can make a very important observation:

**Whenever n is a multiple of 4, the units digit of is **

**1**

That is,

has units digit **1**

has units digit **1**

has units digit **1**

has units digit 1

. . . etc.

At this point, we can find the units digit of

Since 32 is a multiple of 4, must have units digit **1**. From here, we’ll just continue the pattern:

has units digit **1**

has units digit 3

has units digit 9

has units digit 7

The units digit of is 7, which means D is the correct answer to the original question.

For additional practice try these two questions:

1. Find the units digit of .

2. Find the units digit of .

Answers below….

*Answers to the above questions:*

*1. 9*

*2. 4*

## 34 comments

senate on April 2nd, 2013 at 1:46 am

What is the unit digit of 2^2013

Brent Hanneson on April 2nd, 2013 at 7:19 am

2^1 has units digit 2

2^2 has units digit 4

2^3 has units digit 8

2^4 has units digit 6

2^5 has units digit 2

2^6 has units digit 4

2^7 has units digit 8

2^8 has units digit 6

.

.

.

So, the “cycle” equals 4

When the exponent is a multiple of 4, the units digit is 6.

For example 2^4 has units digit 6, 2^8 has units digit 6, etc,

So, 2^2012 has units digit 6, which means 2^2013 has units digit 2,

Cheers,

Brent

talkingdesktop.com on June 7th, 2013 at 8:14 am

What's up, just wanted to tell you, I loved this article. It was funny. Keep on posting!

Alba on July 10th, 2013 at 3:24 pm

It helped me a lot.

Thank you.

Nandy on July 22nd, 2013 at 2:55 am

Really useful.Thank a lot

Brent Hanneson on July 22nd, 2013 at 8:02 am

Thanks for taking the time to provide feedback!

Cheers,

Brent

rahulsethi on July 24th, 2013 at 2:07 am

Wats d answer of (4!)^96 plz help me m stucked

deepchnd on September 20th, 2013 at 6:20 am

6

shifana on July 25th, 2013 at 12:16 am

very goob explanation for unit digit

shifana on July 25th, 2013 at 12:19 am

ver good explanation for unit digit

sumit joshi on August 12th, 2013 at 8:04 am

how to calculate the factorial of any number in an easy methord......

Brent Hanneson on August 12th, 2013 at 8:26 am

I don't believe there exists an easy way to calculate factorials.

Having said that, you shouldn't have to calculate big factorials on the GMAT.

Cheers,

Brent

Sonal on August 24th, 2013 at 2:28 am

Sir, your example is good but i still feel there is very limited explanation provided for every step execution. Also, the pattern forming is difficult to find out as during an exam time. There is always a time limit and one can't spend a lot of time in multiplying values one after the other in order to divulge a patter.

Is there a better option to solve such questions. Please suggest.

Brent Hanneson on August 24th, 2013 at 7:45 am

I'm not aware of a faster approach than what I've described. Since this is a relatively common question type on the GMAT, you will need to become proficient at listing the units digits of various powers.

Cheers,

Brent

deepchnd on September 20th, 2013 at 6:16 am

very much thanq full to u

vishal on October 15th, 2013 at 10:11 am

plz help .....find d unit diigits of

(7^95 - 3^58).....how to solve d substraction? plz explain...

Brent Hanneson on October 15th, 2013 at 10:49 am

7^95 = ---3 and 3^58 = ---9

So, 7^95 - 3^58 = (---3) - (---9) = ---4

Cheers,

Brent

vishal on October 16th, 2013 at 1:15 am

how does dis 4 come?...?.?(---3 ) - (---9) =4???

Brent Hanneson on October 16th, 2013 at 1:04 pm

Test it out.

Take ANY number with units digit 3 and subtract ANY number with units digit 9. What's the units digit of the result?

Cheers,

Brent

jisha on October 17th, 2013 at 6:14 am

Thank you......really useful!!!!

shalini on October 20th, 2013 at 5:57 pm

2^214^302 when divided by 9 .Find the remainder??plz sir explain in detail .finding difficulty in understanding the concept of remainder theorem

Brent Hanneson on October 21st, 2013 at 11:16 am

Hi shalini,

This question requires a knowledge of modular arithmetic (which is not required for the GMAT).

So, unless I'm missing something very basic, this question is out of scope.

Cheers,

Brent

amit on November 1st, 2013 at 5:36 am

Please help me with solving questions like36472^123!

Brent Hanneson on November 1st, 2013 at 10:36 am

Great question. I think you should post it to the Problem Solving forum.

Cheers,

Brent

AJ on November 10th, 2013 at 7:28 pm

For those mentioning time constraints and having to multiply this out, remember, you only really need to multiply the units digit, not the entire number to get the higher powers' units digit. Thanks for writing this, it helped me out so much!

susmitha on January 20th, 2014 at 1:56 am

Wat cud be the maximum value of q in the following equation?

5P9+3r7+2q8=1114

monika on January 21st, 2014 at 2:43 pm

this short trick helped me a lot.

I want to ask u 1 ques.-

87^87 divided by 88

sandeep patel on February 6th, 2014 at 12:47 pm

what is the last digit of139 to the power 99 .can any one answer me.its a CAT question.ans is 7 how is it possible

Brent Hanneson on February 6th, 2014 at 1:02 pm

Hi Sandeep,

Are you sure the answer is 7?

Let's examine a few powers of 139:

139^1 = 139

139^2 = ---1

139^3 = ---9

139^4 = ---1

139^5 = ---9

139^6 = ---1

As we can see, the units digit will equal either 1 or 9

Continuing the pattern, the units digit of 139^99 will be 9

Cheers,

Brent

akshay on February 17th, 2014 at 1:33 pm

how to solve for units place of (15 + root of 220)^82 ?

Brent Hanneson on February 17th, 2014 at 3:10 pm

That question is WAYYYYYY out of scope for the GMAT.

Cheers,

Brent

Rajat on February 23rd, 2014 at 1:24 pm

How about the units digit of factorials, say 25!

Please help.

Brent Hanneson on February 23rd, 2014 at 1:27 pm

For anything past 4!, the units digit will always be 0.

We know this because, when n > 4, n! will contain a 2 and a 5 in its product, which means n! is a multiple of 2 AND a multiple of 5, which means it's a multiple of 10 as well.

All multiples of 10 have 0 as their units digit.

Cheers,

Brent

Vamsee on April 1st, 2014 at 1:22 pm

hello sir...

what is the remainder when 5^2013 is divided by 13?

plzz..

jus explain it to me in detail...!