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# What is the remainder when (18^22)^10 is divided by 7 ?

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Manager
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Joined: 01 Feb 2015
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Concentration: General Management, Economics
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Re: What is the remainder when (18^22)^10 is divided by 7 ? [#permalink]

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13 Dec 2015, 06:26
Financier wrote:
What is the remainder when (18^22)^10 is divided by 7 ?

А. 1
B. 2
C. 3
D. 4
E. 5

Here,
18^22
can be written as 18^(3k+1),k being muliple of 7
Hence (3k+1)^10 div by 7 will yield rem 1 in any case
so,18^1/7
Rem=4
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Joined: 14 Feb 2016
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GMAT 1: 710 Q48 V40
Re: What is the remainder when (18^22)^10 is divided by 7 ? [#permalink]

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15 Feb 2016, 00:47
Bunuel wrote:
Financier wrote:
What is the remainder when (18^22)^10 is divided by 7 ?

А 1
B 2
C 3
D 4
E 5

I think this question is beyond the GMAT scope. It can be solved with Fermat's little theorem, which is not tested on GMAT. Or another way:

$$(18^{22})^{10}=18^{220}=(14+4)^{220}$$ now if we expand this all terms but the last one will have 14 as multiple and thus will be divisible by 7. The last term will be $$4^{220}$$. So we should find the remainder when $$4^{220}$$ is divided by 7.

$$4^{220}=2^{440}$$.

2^1 divided by 7 yields remainder of 2;
2^2 divided by 7 yields remainder of 4;
2^3 divided by 7 yields remainder of 1;

2^4 divided by 7 yields remainder of 2;
2^5 divided by 7 yields remainder of 4;
2^6 divided by 7 yields remainder of 1;
...

So the remainder repeats the pattern of 3: 2-4-1. So the remainder of $$2^{440}$$ divided by 7 would be the same as $$2^2$$ divided by 7 (440=146*3+2). $$2^2$$ divided by 7 yields remainder of 4.

Just remember that all terms in the expression (a+b)^n are divisible by a except for the last term i.e b^n. My solution uses just this much.

(18^22)^10 = (3*3*2)^22^10 = ((7-1)* 3 )^22^10 = [(7-1)^22 * 3^22]^10

(7-1)^10 divided by 7 will have Remainder = 1 ---> A

Now for 3^22 =9^11= (7+2)^11 , the whole term will be divisible by 7 except 2^11

2^11= 2*2* 8^3
2*2* (7+1)^3 or, Remainder =4 ---->B

From statement A and B ,

18^22^10 = (Remainder 1*Remainder4)^10

4^10 = 4 * 64^3 = 4* (63+1)^3

4* remainder 1

I need someone to validate this approach or did I just go absolutely berserk
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Re: What is the remainder when (18^22)^10 is divided by 7 ? [#permalink]

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16 Feb 2016, 21:58
1
KUDOS
Expert's post
KarishmaParmar wrote:
Just remember that all terms in the expression (a+b)^n are divisible by a except for the last term i.e b^n. My solution uses just this much.

I need someone to validate this approach or did I just go absolutely berserk

Yes, that's your binomial theorem concept applied to remainders.
In case you are interested in checking out the details of this approach, look at this post: http://www.veritasprep.com/blog/2011/05 ... ek-in-you/
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Get started with Veritas Prep GMAT On Demand for $199 Veritas Prep Reviews Manager Joined: 14 Feb 2016 Posts: 63 GMAT 1: 710 Q48 V40 What is the remainder when (18^22)^10 is divided by 7 ? [#permalink] ### Show Tags 16 Feb 2016, 22:44 VeritasPrepKarishma wrote: KarishmaParmar wrote: Just remember that all terms in the expression (a+b)^n are divisible by a except for the last term i.e b^n. My solution uses just this much. I need someone to validate this approach or did I just go absolutely berserk Yes, that's your binomial theorem concept applied to remainders. Thanks! I have only started using the concept after reading your post.And now solve all questions using it. Very useful. _________________ It is not who I am underneath but what I do that defines me. Intern Joined: 11 Aug 2016 Posts: 9 Location: United States (PA) Concentration: Finance, General Management GMAT 1: 710 Q48 V40 GPA: 2.42 WE: Other (Mutual Funds and Brokerage) Re: What is the remainder when (18^22)^10 is divided by 7 ? [#permalink] ### Show Tags 24 Sep 2016, 05:44 Just thought I'd share a solution I didn't see here yet: 18^22^10 is the same as (14+4)^22^10, so we can focus on the remainder 4^22^10 when divided by 7. 4^n provides remainders of 4, 2 and 1 when divided by 7 for values of n = 3k+1, 3k+2, and 3k+3 respectively, so we need to find the remainder of 22^10 when divided by 3. 22^10 is the same as (21+1)^10, so we can focus on the remainder of 1^10 when divided by 3, which is 1. Thus, remainder of 4^22^10 is the same as the remainder of 4^(3k+1) which is 4. Therefore our solution is D! Intern Joined: 01 Jun 2013 Posts: 12 What is the remainder when (18^22)^10 is divided by 7 ? [#permalink] ### Show Tags 04 Oct 2016, 11:25 Financier wrote: What is the remainder when (18^22)^10 is divided by 7 ? А. 1 B. 2 C. 3 D. 4 E. 5 =we can express the given expression by ((14+4)^22)^10 and when 14 is divided by 7 remainder is zero so expression reduces to =(4^22)^10= (16^11)^10=((14+2)^11)^10 again after dividing with 7 it reduces to (2^11)^10 =2048^10= (2044+4)^10, when 2048 is divided by 7 remainder is 4, so remaining expression is 4^10. Now when 4 and its exponents are divided by 7 following pattern it shows. Power 1,4,7,10 ------ remainder 4 Power 2,5,8 ------ remainder 2 Power 3,6,9 ------ remainder 1 hence answer is 4. Senior Manager Joined: 13 Oct 2016 Posts: 367 GPA: 3.98 Re: What is the remainder when (18^22)^10 is divided by 7 ? [#permalink] ### Show Tags 28 Nov 2016, 08:39 Financier wrote: What is the remainder when (18^22)^10 is divided by 7 ? А. 1 B. 2 C. 3 D. 4 E. 5 $$\frac{(18^{22})^{10}}{7}$$ $$18 = 4 (mod 7)$$ $$\frac{(4^{22})^{10}}{7} = \frac{4^{220}}{7} = \frac{2^{440}}{7}$$ $$2^3 = 1 (mod 7)$$ $$\frac{2^{438}*2^2}{7} = \frac{(2^3)^{146}*2^2}{7} = \frac{1*4}{7}$$ Remainder is $$4$$. Senior CR Moderator Status: Long way to go! Joined: 10 Oct 2016 Posts: 1328 Location: Viet Nam Re: What is the remainder when (18^22)^10 is divided by 7 ? [#permalink] ### Show Tags 28 Nov 2016, 19:42 Financier wrote: What is the remainder when (18^22)^10 is divided by 7 ? А. 1 B. 2 C. 3 D. 4 E. 5 Quickly solve this question by using modular arithmetic $$\begin{split} 18 &\equiv 4 &\pmod{7} \\ 18^{22} &\equiv 4^{22} = 2^{44} &\pmod{7}\\ (18^{22})^{10} &\equiv (2^{44})^{10} =2^{440} &\pmod{7}\\ \end{split}$$ we have $$\begin{split} 2^3=8 &\equiv 1 &\pmod{7} \\ (2^3)^{146} &\equiv 1 &\pmod{7} \\ 2^{438} &\equiv 1 &\pmod{7} \\ 2^{438} \times 2^2 &\equiv 1 \times 2^2 &\pmod{7} \\ 2^{440} &\equiv 4 &\pmod{7} \\ \implies (18^{22})^{10} &\equiv 4 &\pmod{7} \end{split}$$ The answer is D _________________ Manager Joined: 30 Jun 2015 Posts: 68 Location: Malaysia Schools: Babson '20 GPA: 3.6 Re: What is the remainder when (18^22)^10 is divided by 7 ? [#permalink] ### Show Tags 02 Feb 2018, 13:24 Hi, At this point 2^440/7. 2 has a cycle of 2-4-8-6 and 440 is divisible by 4. thus the units digit = 6 6/7 = remainder = 6.. What am I missing here? Please explain VeritasPrepKarishma wrote: cumulonimbus wrote: Can you point out the mistake here: R(2^440)/7: 2^440 = 2*2^339 = 2*8^113=2*(7+1)^113=2*(7*I+1^113), here I is an integer. R(2*(7*I+1^113))/7 = R(2*7*I+2)/7 = R(2/7) = 2 There's your mistake. If you take a 2 separately, you are left with 439 2s and not 339 2s. $$2^{440} = 4*2^{438} = 4*8^{146} = 4*(7 + 1)^{146}$$ When you divide it by 7, remainder is 4. Veritas Prep GMAT Instructor Joined: 16 Oct 2010 Posts: 7934 Location: Pune, India Re: What is the remainder when (18^22)^10 is divided by 7 ? [#permalink] ### Show Tags 03 Feb 2018, 04:16 cuhmoon wrote: Hi, At this point 2^440/7. 2 has a cycle of 2-4-8-6 and 440 is divisible by 4. thus the units digit = 6 6/7 = remainder = 6.. What am I missing here? Please explain VeritasPrepKarishma wrote: cumulonimbus wrote: Can you point out the mistake here: R(2^440)/7: 2^440 = 2*2^339 = 2*8^113=2*(7+1)^113=2*(7*I+1^113), here I is an integer. R(2*(7*I+1^113))/7 = R(2*7*I+2)/7 = R(2/7) = 2 There's your mistake. If you take a 2 separately, you are left with 439 2s and not 339 2s. $$2^{440} = 4*2^{438} = 4*8^{146} = 4*(7 + 1)^{146}$$ When you divide it by 7, remainder is 4. You are confusing the concept of cyclicity (units digit) with the concept of remainders. When you divide any number which ends in 6 by 7, you do not get a remainder of 6. This works only when you divide a number that ends in 6 by 10. In that case the remainder will be 6 only. e.g. 16 / 7 gives remainder 2 Cyclicity and unit's digit can help you solve remainders questions only in very specific cases. Those are discussed here: https://www.veritasprep.com/blog/2015/1 ... questions/ https://www.veritasprep.com/blog/2015/1 ... ns-part-2/ _________________ Karishma Veritas Prep | GMAT Instructor My Blog Get started with Veritas Prep GMAT On Demand for$199

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What is the remainder when (18^22)^10 is divided by 7 ? [#permalink]

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03 Feb 2018, 07:20
Financier wrote:
What is the remainder when (18^22)^10 is divided by 7 ?

А. 1
B. 2
C. 3
D. 4
E. 5

The equation can be written as $$(18^2)^{110}... (18^2)$$ will give us a remainder of 2...
So, we have now.. $$(2^3)^{36} * 4$$ divided by 7... Remainder = 4
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Re: What is the remainder when (18^22)^10 is divided by 7 ? [#permalink]

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03 Feb 2018, 10:04
Great artciles and very clear explanations! Thanks a lot!

VeritasPrepKarishma wrote:
cuhmoon wrote:
Hi,

At this point 2^440/7.

2 has a cycle of 2-4-8-6 and 440 is divisible by 4. thus the units digit = 6
6/7 = remainder = 6..

What am I missing here? Please explain

You are confusing the concept of cyclicity (units digit) with the concept of remainders.
When you divide any number which ends in 6 by 7, you do not get a remainder of 6. This works only when you divide a number that ends in 6 by 10. In that case the remainder will be 6 only.

e.g. 16 / 7 gives remainder 2

Cyclicity and unit's digit can help you solve remainders questions only in very specific cases. Those are discussed here:
https://www.veritasprep.com/blog/2015/1 ... questions/
https://www.veritasprep.com/blog/2015/1 ... ns-part-2/
Re: What is the remainder when (18^22)^10 is divided by 7 ?   [#permalink] 03 Feb 2018, 10:04

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