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gurpreetsingh
Similar question to test what you have learnt from the previous post.

What is the remainder when \(32^{32^{32}}\) is divided by 9?

A. 7
B. 4
C. 2
D. 0
E. 1


32^{32^{32}}=(27+5)^{32^{32}}
Solving this binomial power only the last term won't be divisible by 9, so let's consider only this term:
5^{32^{32}}=5^{2^{5*32}}=5^{2^{160}}



Let's consider ciclicity when a power of 5 is divided by 9 (NO WAY I COULD DO THIS ON TEST DAY!!!):
- 5^1 divided by 9 gives a reminder of 5
- 5^2 divided by 9 gives a reminder of 7
- 5^3 divided by 9 gives a reminder of 8
- 5^4 divided by 9 gives a reminder of 4
- 5^5 divided by 9 gives a reminder of 2
- 5^6 divided by 9 gives a reminder of 1

- 5^7 divided by 9 gives a reminder of 5
- 5^8 divided by 9 gives a reminder of 7
- 5^9 divided by 9 gives a reminder of 8
- 5^10 divided by 9 gives a reminder of 4
...

Therefore ciclicity is 6.
Let's divide our exponent 2^{160} by 6 and figure out the reminder.



Ciclicty for power of 2 divided by 6 is:
- 2^1 divided by 6 gives a reminder of 2
- 2^2 divided by 6 gives a reminder of 4

- 2^3 divided by 6 gives a reminder of 2
- 2^4 divided by 6 gives a reminder of 4
...

For even exponents the reminder is always 4 (use it to go back in the previous ciclicity and pick the right number).

THE REMINDER IS 4

CORRECT ANS IS B!!
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Bunuel
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What is the remainder when \(32^{32^{32}}\) is divided by 7?

A. 5
B. 4
C. 2
D. 0
E. 1

If we use the above approach I'd work with prime as a base.

\(32^{{32}^{32}}=(28+4)^{{32}^{32}}\) now if we expand this, all terms but the last one will have 28 as a multiple and thus will be divisible by 7. The last term will be \(4^{{32}^{32}}=4^{{(2^5)}^{32}}=4^{2^{160}}=2^{2^{161}}\). So we should find the remainder when \(2^{2^{161}}\) is divided by 7.

Hope it's clear.

VeritasKarishma

Can you pls explain how we got the highlighted portion, 4 can be written as 2^2 and that would translate to 2^4^160

Pls explain
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Bunuel
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What is the remainder when \(32^{32^{32}}\) is divided by 7?

A. 5
B. 4
C. 2
D. 0
E. 1

If we use the above approach I'd work with prime as a base.

\(32^{{32}^{32}}=(28+4)^{{32}^{32}}\) now if we expand this, all terms but the last one will have 28 as a multiple and thus will be divisible by 7. The last term will be \(4^{{32}^{32}}=4^{{(2^5)}^{32}}=4^{2^{160}}=2^{2^{161}}\). So we should find the remainder when \(2^{2^{161}}\) is divided by 7.

Hope it's clear.

VeritasKarishma

Can you pls explain how we got the highlighted portion, 4 can be written as 2^2 and that would translate to 2^4^160

Pls explain

\(4^{2^{160}}=(2^2)^{2^{160}}= 2^{2*2^{160}}=2^{2^{160+1}}=2^{2^{161}}\)
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I broke down the question to 2^30/7
Now if we directly use the cyclicity of 2, we get 4 as the remainder.
But if i break it in terms of 8 ie 2 ^3 like (2^3)^10/7, we get remainder as 1. How would we know which is right and which is wrong?
Bunuel chetan2u VeritasKarishma
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TaizKaran
I broke down the question to 2^30/7
Now if we directly use the cyclicity of 2, we get 4 as the remainder.
But if i break it in terms of 8 ie 2 ^3 like (2^3)^10/7, we get remainder as 1. How would we know which is right and which is wrong?
Bunuel chetan2u VeritasKarishma

TaizKaran:

Units digit cyclicity DOES NOT help you with remainders except in cases when the divisor is 2/5/10.

Note that
26/7 Remainder = 5,
36/7 Remainder = 1,
46/7 Remainder = 4
So even though they all have same units digit of 6, the remainders are different in each case.

You need to use binomial as done by you here "(2^3)^10/7, we get remainder as 1"

See:
https://www.gmatclub.com/forum/veritas-prep-resource-links-no-longer-available-399979.html#/2015/1 ... questions/
https://www.gmatclub.com/forum/veritas-prep-resource-links-no-longer-available-399979.html#/2015/1 ... ns-part-2/
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Hi Bunuel chetan2u,

I have a question with respect to the simplification of \((4)^{2^1^6^0^}\). I understand how you got \((2)^{2^1^6^1}\).
However, I don't get the same result when I simplify \((4)^{2^1^6^0^}\) as follows: \((2 * 2)^{2^1^6^0}\) => \((2)^{2^1^6^0}\) * \((2)^{2^1^6^0}\) => \((2)^{2^(^1^6^0+^1^6^0^)}\)

Please can you help me understand my mistake?

Thanks a lot
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Hi Bunuel chetan2u,

I have a question with respect to the simplification of \((4)^{2^1^6^0^}\). I understand how you got \((2)^{2^1^6^1}\).
However, I don't get the same result when I simplify \((4)^{2^1^6^0^}\) as follows: \((2 * 2)^{2^1^6^0}\) => \((2)^{2^1^6^0}\) * \((2)^{2^1^6^0}\) => \((2)^{2^(^1^6^0+^1^6^0^)}\)

Please can you help me understand my mistake?

Thanks a lot


\((2)^{2^{160}}*(2)^{2^{160}}=2^{(2^{160}+2^{160})}=(2)^{(2*2^{160})}=2^{2^{161}}\)
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\(32 ^{32}\) is always even.

and 32 divided by 7 will give remainder as 4.

\(4^{even power}\) , remainder will be 4.

Answer B
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Bunuel

gurpreetsingh
What is the remainder when \(32^{32^{32}}\) is divided by 7?

A. 5
B. 4
C. 2
D. 0
E. 1

Please do not just post the answer, do explain as well.

I will post the Answer and the explanation after some replies.
If we use the above approach I'd work with prime as a base.

\(32^{{32}^{32}}=(28+4)^{{32}^{32}}\) now if we expand this, all terms but the last one will have 28 as a multiple and thus will be divisible by 7. The last term will be \(4^{{32}^{32}}=4^{{(2^5)}^{32}}=4^{2^{160}}=2^{2^{161}}\). So we should find the remainder when \(2^{2^{161}}\) is divided by 7.

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;
...

The remainder repeats the pattern of 3: 2-4-1.

So we should find \(2^{161}\) (the power of 2) is 1st, 2nd or 3rd number in the above pattern of 3. \(2^{161}\) is 2 in odd power, 2 in odd power gives remainder of 2 when divided by cyclicity number 3, so it's the second number in pattern. Which means that remainder of \(2^{2^{161}}\) divided by 7 would be the same as \(2^2\) divided by 7. \(2^2\) divided by 7 yields remainder of 4.

Answer: B.

Similar problem: https://gmatclub.com/forum/remainder-99 ... ms#p768816

Hope it's clear.
­Hello Bunuel, could you explain how \( 4^{2^{160}}=2^{2^{161}} \) this happens?
Many thanks!
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lnyngayan


­Hello Bunuel, could you explain how \( 4^{2^{160}}=2^{2^{161}} \) this happens?
Many thanks!
­
\(4^{2^{160}}=(2^2)^{2^{160}}= 2^{2*2^{160}}=2^{2^{160+1}}=2^{2^{161}}\)­
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We need to find the remainder when \(32^{32^{32}}\) is divided by 7?

\(32^{32^{32}}\)= \(32^{2^{5*32}}\) = \(32^{2^{160}}\)

We solve these problems by using Binomial Theorem, where we split the number into two parts, one part is a multiple of the divisor(7) and a big number, other part is a small number.

=> \(32^{2^{160}}\) = \((28+4)^{2^{160}}\)

Watch this video to MASTER BINOMIAL Theorem

Now, if we use Binomial Theorem to expand this then all the terms except the last term will be a multiple of 7
=> All terms except the last term will give remainder of 0 when divided by 7

=> Problem is reduced to what is the remainder when the last term (i.e. 2^160 C 2^160 * 28^0 * \(4^{2^{160}}\)) is divided by 7
=> Remainder of \(4^{2^{160}}\) is divided by 7
=> Remainder of \(2^{2^1 * 2^{160}}\) is divided by 7
=> Remainder of \(2^{2^{161}}\) is divided by 7

To solve this problem we need to find the cycle of remainder of power of 2 when divided by 7

Remainder of \(2^1\) (=2) by 7 = 2
Remainder of \(2^2\) (=4) by 7 = 4
Remainder of \(2^3\) (=8) by 7 = 1
Remainder of \(2^4\) (=16) by 7 = 2
Remainder of \(2^5\) (=32) by 7 = 4
Remainder of \(2^6\) (=64) by 7 = 1

=> Cycle is 3

We need to find Remainder of \(2^{161}\) by 3

Remainder of \(2^1\) (=2) by 3 = 2
Remainder of \(2^2\) (=4) by 3 = 1
Remainder of \(2^3\) (=8) by 3 = 2
Remainder of \(2^4\) (=16) by 3 = 1

=> Cycle is 2

=> Remainder of \(2^{161}\) by 3 = Remainder of \(2^{1}\) by 3 = 2
=> Remainder of \(2^{2^{161}}\) is divided by 7 = Remainder of 2^2 by 7 = 4

So, Answer will be B
Hope it Helps!

Watch following video to MASTER Remainders by 2, 3, 5, 9, 10 and Binomial Theorem

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Honestly, This question wasn't difficult at all IMHO. It should not be in "Out of Scope- Too Hard" section of Gmatclub.
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What is the remainder when \(32^{32^{32}}\) is divided by 7?

The remainder when \(32^{32^{32}}\) is divided by 7
= The remainder when \((-3)^{32^{32}}\) is divided by 7
= The remainder when \(3^{32^{32}}\) is divided by 7
= The remainder when \({3^{3*10+2}}^{32}\) is divided by 7
= The remainder when \(3^{2*32}\) is divided by 7
= The remainder when \(3^{3*21+1}\) is divided by 7
= The remainder when \((-1)^{21}*3\) is divided by 7
= The remainder when -3 is divided by 7
=7-3 = 4

IMO B
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