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# What is the remainder when 7^8 is divided by 100?

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Math Revolution GMAT Instructor
Joined: 16 Aug 2015
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What is the remainder when 7^8 is divided by 100?  [#permalink]

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26 Jul 2018, 00:47
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82% (00:49) correct 18% (01:21) wrong based on 93 sessions

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[Math Revolution GMAT math practice question]

What is the remainder when $$7^8$$ is divided by $$100$$?

A. $$1$$
B. $$2$$
C. $$3$$
D. $$4$$
E. $$5$$

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"Only $99 for 3 month Online Course" "Free Resources-30 day online access & Diagnostic Test" "Unlimited Access to over 120 free video lessons - try it yourself" Intern Joined: 17 May 2018 Posts: 38 Re: What is the remainder when 7^8 is divided by 100? [#permalink] ### Show Tags 26 Jul 2018, 00:56 Typical example of big exponents, where we need to find the pattern for the last digit. 7^1=7 7^2=49 7^3 last digit will be last digit of 9x7=63 => 3 7^4 last digit will be last digit of 3x7=21 =>1 7^5 last digit will be last digit of 1x7=7 => 7 7^6 last digit will be last digit of 7x7=49 => 9 now the pattern repeats itself 7^7 last digit will be 3 7^8 last digit will be 1 Answer A _________________ ¿Tienes que presentar el GMAT y no sabes por dónde empezar? ¡Visita GMAT para Principiantes y recibe el curso completo gratis! Senior Manager Joined: 04 Aug 2010 Posts: 306 Schools: Dartmouth College Re: What is the remainder when 7^8 is divided by 100? [#permalink] ### Show Tags 26 Jul 2018, 02:32 MathRevolution wrote: [Math Revolution GMAT math practice question] What is the remainder when $$7^8$$ is divided by $$100$$? A. $$1$$ B. $$2$$ C. $$3$$ D. $$4$$ E. $$5$$ When an integer is divided by 100, the remainder will have the same units digit as the integer. Thus, to determine which answer choice represents the remainder when $$7^8$$ is divided by 100, we need to know the units digit of $$7^8$$. When an integer is raised to consecutive powers, the resulting units digits repeat in a CYCLE. $$7^1$$ --> units digit of 7. $$7^2$$ --> units digit of 9. (Since the product of the preceding units digit and 7 = 7*7 = 49.) $$7^3$$ --> units digit of 3. (Since the product of the preceding units digit and 7 = 9*7 = 63.) $$7^4$$ --> units digit of 1. (Since the product of the preceding units digit and 7 = 3*7 = 21.) From here, the units digits will repeat in the same pattern: 7, 9, 3, 1. The units digit repeat in a CYCLE OF 4. Implication: When an integer with a units digit of 7 is raised to a power that is a multiple of 4, the units digit will be 1. Thus, $$7^8$$ has a units digit of 1. _________________ GMAT and GRE Tutor Over 1800 followers Click here to learn more GMATGuruNY@gmail.com New York, NY If you find one of my posts helpful, please take a moment to click on the "Kudos" icon. Available for tutoring in NYC and long-distance. For more information, please email me at GMATGuruNY@gmail.com. CEO Joined: 11 Sep 2015 Posts: 3122 Location: Canada Re: What is the remainder when 7^8 is divided by 100? [#permalink] ### Show Tags 26 Jul 2018, 05:21 Top Contributor MathRevolution wrote: [Math Revolution GMAT math practice question] What is the remainder when $$7^8$$ is divided by $$100$$? A. $$1$$ B. $$2$$ C. $$3$$ D. $$4$$ E. $$5$$ Let's examine 7^8 - 1 Why would I do this? Well, I know that 7^2 + 1 = 50, which is a factor of 100. So, perhaps it's the case that 7^8 - 1 is divisible by 100, in which case 7^8 will leave a remainder of 1 when divided by 100 7^8 - 1 is a difference of squares. So, 7^8 - 1 = (7^4 + 1)(7^4 - 1) = (7^4 + 1)(7^2 + 1)(7^2 - 1) = (7^4 + 1)(7^2 + 1)(7 + 1)(7 - 1) = (7^4 + 1)(50)(8)(6) = (7^4 + 1)(2400) = (7^4 + 1)(24)(100) So, we can see that 7^8 - 1 is divisible by 100 7^8 is 1 greater than 7^8 - 1, so we must get a remainder of 1 when 7^8 is divided by 100 Answer: A Cheers, Brent _________________ Test confidently with gmatprepnow.com Senior Manager Joined: 08 Aug 2017 Posts: 252 Re: What is the remainder when 7^8 is divided by 100? [#permalink] ### Show Tags 26 Jul 2018, 06:44 1 I did this in following way. 49^4*2^4/2^4*100 = 98^4/2^4*100 =(-2)^4/2^4 = 1 That's the answer. Please appraise my approach and give kudos if I am correct on my way. Board of Directors Status: QA & VA Forum Moderator Joined: 11 Jun 2011 Posts: 4220 Location: India GPA: 3.5 WE: Business Development (Commercial Banking) Re: What is the remainder when 7^8 is divided by 100? [#permalink] ### Show Tags 26 Jul 2018, 10:05 MathRevolution wrote: [Math Revolution GMAT math practice question] What is the remainder when $$7^8$$ is divided by $$100$$? A. $$1$$ B. $$2$$ C. $$3$$ D. $$4$$ E. $$5$$ $$\frac{7^4}{100}$$ = Remainder 1 Thus, $$\frac{7^8}{100}$$ = Remainder 1, Hence answer must be (A) _________________ Thanks and Regards Abhishek.... PLEASE FOLLOW THE RULES FOR POSTING IN QA AND VA FORUM AND USE SEARCH FUNCTION BEFORE POSTING NEW QUESTIONS How to use Search Function in GMAT Club | Rules for Posting in QA forum | Writing Mathematical Formulas |Rules for Posting in VA forum | Request Expert's Reply ( VA Forum Only ) Study Buddy Forum Moderator Joined: 04 Sep 2016 Posts: 1253 Location: India WE: Engineering (Other) Re: What is the remainder when 7^8 is divided by 100? [#permalink] ### Show Tags 28 Jul 2018, 07:14 pkd niks18 chetan2u KarishmaB gmatbusters Quote: When an integer is divided by 100, the remainder will have the same units digit as the integer. I could not recall such a rule under timing pressure. Although w/o clock I figured out that if I take random number say 202 and divide it by 100 I get remainder as 2, which is same as unit digits of 202. My mind intuitively went into a decimal form of integers under timed stress e.g. 202/100 = 2.02 and I could not relate cyclicity to a decimal form of a fraction. Any two cents here to relate divisibility by 100 with unit digits? _________________ It's the journey that brings us happiness not the destination. Math Expert Joined: 02 Aug 2009 Posts: 7036 Re: What is the remainder when 7^8 is divided by 100? [#permalink] ### Show Tags 28 Jul 2018, 07:38 1 adkikani wrote: pkd niks18 chetan2u KarishmaB gmatbusters Quote: When an integer is divided by 100, the remainder will have the same units digit as the integer. I could not recall such a rule under timing pressure. Although w/o clock I figured out that if I take random number say 202 and divide it by 100 I get remainder as 2, which is same as unit digits of 202. My mind intuitively went into a decimal form of integers under timed stress e.g. 202/100 = 2.02 and I could not relate cyclicity to a decimal form of a fraction. Any two cents here to relate divisibility by 100 with unit digits? It is also true for divisibility by 10.. Reason is if the number has to be divisible by 10 or 100, it has to have 0 as units digit. So whatever on top of 0 will be units digit.. 202/100= (200+2)/100=200/100+2/100.. So 200 is div and left is 2 _________________ 1) Absolute modulus : http://gmatclub.com/forum/absolute-modulus-a-better-understanding-210849.html#p1622372 2)Combination of similar and dissimilar things : http://gmatclub.com/forum/topic215915.html 3) effects of arithmetic operations : https://gmatclub.com/forum/effects-of-arithmetic-operations-on-fractions-269413.html GMAT online Tutor PS Forum Moderator Joined: 25 Feb 2013 Posts: 1217 Location: India GPA: 3.82 Re: What is the remainder when 7^8 is divided by 100? [#permalink] ### Show Tags 28 Jul 2018, 08:29 1 adkikani wrote: pkd niks18 chetan2u KarishmaB gmatbusters Quote: When an integer is divided by 100, the remainder will have the same units digit as the integer. I could not recall such a rule under timing pressure. Although w/o clock I figured out that if I take random number say 202 and divide it by 100 I get remainder as 2, which is same as unit digits of 202. My mind intuitively went into a decimal form of integers under timed stress e.g. 202/100 = 2.02 and I could not relate cyclicity to a decimal form of a fraction. Any two cents here to relate divisibility by 100 with unit digits? Hi adkikani The basic problem that you discussed here is that you went on to think of a decimal number for a remainder question. In most "Remainder" type question in GMAT you need not think of a decimal number. The options for these questions are also pretty straight forward integers. So as a thumb rule when you see a remainder question you should think of D=Q+R form, where R is the remainder. and the cyclicity and properties of 10 and its powers are pretty simple to learn. You can practice few questions to internalize it Veritas Prep GMAT Instructor Joined: 16 Oct 2010 Posts: 8550 Location: Pune, India Re: What is the remainder when 7^8 is divided by 100? [#permalink] ### Show Tags 28 Jul 2018, 23:58 1 adkikani wrote: pkd niks18 chetan2u KarishmaB gmatbusters Quote: When an integer is divided by 100, the remainder will have the same units digit as the integer. I could not recall such a rule under timing pressure. Although w/o clock I figured out that if I take random number say 202 and divide it by 100 I get remainder as 2, which is same as unit digits of 202. My mind intuitively went into a decimal form of integers under timed stress e.g. 202/100 = 2.02 and I could not relate cyclicity to a decimal form of a fraction. Any two cents here to relate divisibility by 100 with unit digits? Another thing - remainders and decimals are two different (but equivalent of course) ways in which you can show the output of a division. e.g.13/5 => 2 quotient and 3 remainder or 13/5 => 2.6 (the integer part corresponds to the quotient and 6/10 = 3/5 = remainder/divisor) So when talking about remainders, we are looking at the result of division from the quotient/remainder perspective, not from the decimal perspective. As for the connection between units digit and division by 2/5/10 or a multiple of 10, check out the two posts I wrote for Veritas Prep: https://www.veritasprep.com/blog/2015/1 ... questions/ https://www.veritasprep.com/blog/2015/1 ... ns-part-2/ _________________ Karishma Veritas Prep GMAT Instructor Learn more about how Veritas Prep can help you achieve a great GMAT score by checking out their GMAT Prep Options > GMAT self-study has never been more personalized or more fun. Try ORION Free! Math Revolution GMAT Instructor Joined: 16 Aug 2015 Posts: 6517 GMAT 1: 760 Q51 V42 GPA: 3.82 Re: What is the remainder when 7^8 is divided by 100? [#permalink] ### Show Tags 29 Jul 2018, 17:34 => The remainder when $$7^8$$ is divided by $$100$$ is equal to the final two digits of $$7^8$$. Now, $$7^1 = 7, 7^2 = 49,$$ $$7^3 = 343$$, and $$7^4 = 2401$$. So, the final two digits of $$7^n$$ have period $$4$$: The tens digits are $$0 -> 4 -> 4 -> 0$$ and the units digits are $$7 -> 9 -> 3 -> 1.$$ It follows that the tens and units digits of $$7^8$$ are $$0$$ and $$1$$, respectively. Therefore, the remainder when $$7^8$$ is divided by $$100$$ is $$1$$. Therefore, the answer is A. Answer : A _________________ MathRevolution: Finish GMAT Quant Section with 10 minutes to spare The one-and-only World’s First Variable Approach for DS and IVY Approach for PS with ease, speed and accuracy. "Only$99 for 3 month Online Course"
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Re: What is the remainder when 7^8 is divided by 100? &nbs [#permalink] 29 Jul 2018, 17:34
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