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# What is the units digit of 31467^32 × 97645^23 × (32168^5 + 8652)^479?

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Joined: 17 Oct 2016
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What is the units digit of 31467^32 × 97645^23 × (32168^5 + 8652)^479? [#permalink]

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09 Aug 2017, 19:51
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Difficulty:

25% (medium)

Question Stats:

79% (00:58) correct 21% (00:52) wrong based on 40 sessions

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What is the units digit of 31467^32 × 97645^23 × (32168^5 + 8652)^479?

A. 0

B. 1

C. 2

D. 5

E. 8
[Reveal] Spoiler: OA

_________________

Sometimes you have to burn yourself to the ground before you can rise like a phoenix from the ashes.

Dr. Pratik

Last edited by Bunuel on 10 Aug 2017, 00:55, edited 2 times in total.
Renamed the topic.

Kudos [?]: 33 [0], given: 17

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Joined: 02 Aug 2009
Posts: 4967

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Re: What is the units digit of 31467^32 × 97645^23 × (32168^5 + 8652)^479? [#permalink]

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09 Aug 2017, 23:47
1
KUDOS
Expert's post
fitzpratik wrote:
What is the units digit of 314673^2 × 976452^3 × (32168^5 + 865^2)^479?

A. 0

B. 1

C. 2

D. 5

E. 8

Hi...
Relook at the Q and choices, 0 can never be the units digit here..

For ANSWER let's see only the units digit..
$$3^2*2^3*(8^5+5^2)^{479}$$
... This means $$9*8*(8+5)^{480-1}$$
480 is div by 4 so units digit of base will be same as that of 4-1 or 3..
So 72*13^3.....2*3^3=2*7=14..
Units digit is 4
_________________

Absolute modulus :http://gmatclub.com/forum/absolute-modulus-a-better-understanding-210849.html#p1622372
Combination of similar and dissimilar things : http://gmatclub.com/forum/topic215915.html

Kudos [?]: 5453 [1], given: 112

Manager
Joined: 17 Oct 2016
Posts: 81

Kudos [?]: 33 [0], given: 17

Location: India
Concentration: General Management, Healthcare
GMAT 1: 640 Q40 V38
GPA: 3.05
WE: Pharmaceuticals (Health Care)
Re: What is the units digit of 31467^32 × 97645^23 × (32168^5 + 8652)^479? [#permalink]

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09 Aug 2017, 23:57
chetan2u wrote:
fitzpratik wrote:
What is the units digit of 314673^2 × 976452^3 × (32168^5 + 865^2)^479?

A. 0

B. 1

C. 2

D. 5

E. 8

Hi...
Relook at the Q and choices, 0 can never be the units digit here..

For ANSWER let's see only the units digit..
$$3^2*2^3*(8^5+5^2)^{479}$$
... This means $$9*8*(8+5)^{480-1}$$
480 is div by 4 so units digit of base will be same as that of 4-1 or 3..
So 72*13^3.....2*3^3=2*7=14..
Units digit is 4

Hi chetan2u

Thank you.
_________________

Sometimes you have to burn yourself to the ground before you can rise like a phoenix from the ashes.

Dr. Pratik

Kudos [?]: 33 [0], given: 17

Math Forum Moderator
Joined: 02 Aug 2009
Posts: 4967

Kudos [?]: 5453 [0], given: 112

Re: What is the units digit of 31467^32 × 97645^23 × (32168^5 + 8652)^479? [#permalink]

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10 Aug 2017, 00:46
fitzpratik wrote:
What is the units digit of 31467^32 × 97645^23 × (32168^5 + 8652)^479?

A. 0

B. 1

C. 2

D. 5

E. 8

For ANSWER let's see only the units digit..
$$7^{32}*5^{23}*(8^5+2)^{479}$$
... This means $$7^4*5*(8^1+2)^{480-1}$$......
after every 4th power, the units digit repeat
for units digit 0, 1, 5 and 6... it remains the same for every positive integer power
for 2,3,7,8 it repeats after every 4th.. so 2^401 will be same as 2^(4*50+1) or 2^1
4 and 9 repeat after every second..

so $$7^4*5$$ will be 5 as ODD *5 will have units digit 5
but $$(8^1+2)^{479} = 10^{479}$$ will always have 0 as units digit..
so overall also it will be 0

A
_________________

Absolute modulus :http://gmatclub.com/forum/absolute-modulus-a-better-understanding-210849.html#p1622372
Combination of similar and dissimilar things : http://gmatclub.com/forum/topic215915.html

Kudos [?]: 5453 [0], given: 112

Manager
Joined: 17 Oct 2016
Posts: 81

Kudos [?]: 33 [0], given: 17

Location: India
Concentration: General Management, Healthcare
GMAT 1: 640 Q40 V38
GPA: 3.05
WE: Pharmaceuticals (Health Care)
Re: What is the units digit of 31467^32 × 97645^23 × (32168^5 + 8652)^479? [#permalink]

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10 Aug 2017, 01:18
chetan2u wrote:
fitzpratik wrote:
What is the units digit of 31467^32 × 97645^23 × (32168^5 + 8652)^479?

A. 0

B. 1

C. 2

D. 5

E. 8

For ANSWER let's see only the units digit..
$$7^{32}*5^{23}*(8^5+2)^{479}$$
... This means $$7^4*5*(8^1+2)^{480-1}$$......
after every 4th power, the units digit repeat
for units digit 0, 1, 5 and 6... it remains the same for every positive integer power
for 2,3,7,8 it repeats after every 4th.. so 2^401 will be same as 2^(4*50+1) or 2^1
4 and 9 repeat after every second..

so $$7^4*5$$ will be 5 as ODD *5 will have units digit 5
but $$(8^1+2)^{479} = 10^{479}$$ will always have 0 as units digit..
so overall also it will be 0

A

Could not have put it in any better way!!
_________________

Sometimes you have to burn yourself to the ground before you can rise like a phoenix from the ashes.

Dr. Pratik

Kudos [?]: 33 [0], given: 17

Director
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Posts: 802

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What is the units digit of 31467^32 × 97645^23 × (32168^5 + 8652)^479? [#permalink]

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11 Aug 2017, 07:32
chetan2u wrote:
fitzpratik wrote:
What is the units digit of 31467^32 × 97645^23 × (32168^5 + 8652)^479?

A. 0

B. 1

C. 2

D. 5

E. 8

For ANSWER let's see only the units digit..
$$7^{32}*5^{23}*(8^5+2)^{479}$$
... This means $$7^4*5*(8^1+2)^{480-1}$$......
after every 4th power, the units digit repeat
for units digit 0, 1, 5 and 6... it remains the same for every positive integer power
for 2,3,7,8 it repeats after every 4th.. so 2^401 will be same as 2^(4*50+1) or 2^1
4 and 9 repeat after every second..

so $$7^4*5$$ will be 5 as ODD *5 will have units digit 5
but $$(8^1+2)^{479} = 10^{479}$$ will always have 0 as units digit..
so overall also it will be 0

A

chetan2u , I'm not clear about why you wrote this part: $$(8^1+2)^{480-1}$$. (480/4) is evenly divisible, maybe the (-1) is a negative remainder (and I will never understand negative remainders fully) but that doesn't seem to explain the exponent manipulation, either. . .

Was it to show remainder in case $$(8^1+2)$$ summed to something other than zero? If that is the reason, I still don't follow.

You can't distribute the exponent, so cyclicity of four for addends 8 and 2 can't be the reason.

And cyclicity per se doesn't seem to be the reason: 2, 3, 7, and 8 have cyclicity of four (where 480 is divisible by 4) - but 0, 1, 4, 5, 6, and 9 do not have cyclicity of four. That 8 might have had units digit 2, e.g. Then units digit is 4, and I can't see how $$4^{480-1}$$ helps.

The (-1) part isn't clear, either. I suppose I know that, in the number whose cyclicity of powers is four, I can take the $$n^4$$ term and "move backwards" by one power in the cycle to get the units digit, but 479/4 leaves remainder 3, which is the same thing as "moving one place backward" in the cycle. It seems harder to see, from a divisibility perspective, that (-1) power in cycle = (+3) power in cycle.

I looked only at $$8^5$$, where 5/4 = remainder 1, units digit of first term is same as $$8^1$$ = 8. Last digit of addend in parentheses is 2. (8 + 2) = 0.

And as you point out, zero to any power (except zero) is 0. So I just wrote (first expression) * 0.

Sorry if this confusion arises because I am missing something obvious. I suspect what you wrote is important for problems different from this one. Please explain?

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What is the units digit of 31467^32 × 97645^23 × (32168^5 + 8652)^479?   [#permalink] 11 Aug 2017, 07:32
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