What is the remainder, after division by 100, of 7^10 ? : GMAT Problem Solving (PS)
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What is the remainder, after division by 100, of 7^10 ?

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What is the remainder, after division by 100, of 7^10 ? [#permalink]

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07 Oct 2011, 20:45
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What is the remainder, after division by 100, of 7^10 ?

(A) 1
(B) 7
(C) 43
(D) 49
(E) 70
[Reveal] Spoiler: OA

Last edited by Bunuel on 14 Dec 2012, 01:13, edited 2 times in total.
Renamed the topic and edited the question.
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07 Oct 2011, 22:29
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We have such a pattern
7
7*7=49
7*7*7=343
7*7*7*7=...1
7*7*7*7*7=7
So, the last number repeats every 5th time.
10/4 = 2, and remainder is 2.
we choose 7*7
it means that 7*7*7*7*7*7*7*7*7*7=.......49
We divide by 100, it means .....,49
where 49 is a remainder.
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09 Oct 2011, 05:02
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kkalyan wrote:
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Help End Exam Review Section

What is the remainder, after division by 100, of $$7^{10}$$ ?

1
7
43
49
70

PLZ EXPLAIN THE TRICK IN SOLVING THIS TYPE REMAINDER PROBLEM

Using Binomial theorem, last two digits of an exponent can be found as
7^(10)=7^(2*5)=49^5=(-1+50)^5=(-1)^5+5*(-1)^4*50=-1+50(Just considered last 2-digit of the product)=49

Look for Karishma's blogs. You may find more.

Ans: "D"
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Re: PS: Remainder of 7^(10) divided by 100 [#permalink]

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09 Oct 2011, 08:35
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kkalyan wrote:
What is the remainder, after division by 100, of $$7^{10}$$ ?

(A) 1
(B) 7
(C) 43
(D) 49
(E) 70

7^4 is 2401 .

So we can write 7^10 as ( 7^4 *7^4 * 7^2) divided by 100 ..

This would give us (1*1*49)/ 100 which would give remainder as 49.
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Re: PS: Remainder of 7^(10) divided by 100 [#permalink]

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14 Dec 2012, 00:48
if we do not hav formular, how do I do?
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Re: PS: Remainder of 7^(10) divided by 100 [#permalink]

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14 Dec 2012, 01:21
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thangvietnam wrote:
if we do not hav formular, how do I do?

What is the remainder, after division by 100, of 7^10 ?

(A) 1
(B) 7
(C) 43
(D) 49
(E) 70

The remainder when 7^10 is divided by 100 will be the last two digits of 7^10 (for example 123 divided by 100 yields the remainder of 23, 345 divided by 100 yields the remainder of 45).

$$7^{10}=(7^2)^5=49^5$$ --> the units digit of 49^5 will be 9 (the units digit of 9^even is 1 and the units digit of 9^odd is 9).

So, we have that $$7^{10}=49^5$$ has the units digit of 9, thus the units digit of the remainder must also be 9. Only answer D fits.

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Re: What is the remainder, after division by 100, of 7^10 ? [#permalink]

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16 Dec 2012, 12:56
Hey Bunnel,

this rule is general ? like if XXXXXX9^even the unit digit of the remainder is always 1 and XXXXX9^odd the unit digit of the remainder is always 9 ??

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Re: What is the remainder, after division by 100, of 7^10 ? [#permalink]

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16 Dec 2012, 22:39
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see wrote:
Hey Bunnel,

this rule is general ? like if XXXXXX9^even the unit digit of the remainder is always 1 and XXXXX9^odd the unit digit of the remainder is always 9 ??

The units digit of 9^even is 1 and the units digit of 9^odd is 9.

If the units digit of a number is 1, then the remainder when this number will be divided by 100 will have the units digit of 1, for example 231 divided by 100 gives the reminder of 31.

If the units digit of a number is 9, then the remainder when this number will be divided by 100 will have the units digit of 9, for example 239 divided by 100 gives the reminder of 39.
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Re: What is the remainder, after division by 100, of 7^10 ? [#permalink]

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17 Dec 2012, 23:14
what if the answer choise has another value with units digit 9? how do we need to proceed in that case?
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Re: What is the remainder, after division by 100, of 7^10 ? [#permalink]

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18 Dec 2012, 00:38
Ans:

7^10 can be written as 49^5 which can be written as (49^2)^2. 49 when divided by 100 it will give a remainder of (1)^2.49=49 answer (D).
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Re: What is the remainder, after division by 100, of 7^10 ? [#permalink]

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04 Feb 2013, 03:11
7 ^1 has last digit is 7
7^2 has last digit is 9
3
1

the last digit of 7^10 must be 9

the remainder must has the same last digit

only D fits
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Re: What is the remainder, after division by 100, of 7^10 ? [#permalink]

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17 Dec 2013, 05:59
priyamne wrote:
Ans:

7^10 can be written as 49^5 which can be written as (49^2)^2. 49 when divided by 100 it will give a remainder of (1)^2.49=49 answer (D).

49^5 is not (49^2)^2

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Re: What is the remainder, after division by 100, of 7^10 ? [#permalink]

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01 May 2014, 02:10
Amateur wrote:
what if the answer choise has another value with units digit 9? how do we need to proceed in that case?

Good question. Any clues Bunnuel
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Re: What is the remainder, after division by 100, of 7^10 ? [#permalink]

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17 Sep 2014, 10:13
kkalyan wrote:
What is the remainder, after division by 100, of 7^10 ?

(A) 1
(B) 7
(C) 43
(D) 49
(E) 70

sol:

7=7
7^2=..9
7^3=..3
7^4=1

now 10/4= 2 i.e. second from top of the pattern...which is 9
since we are dividing the number by 100 last number will be reminder
check the answers....D is the only choice
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Re: What is the remainder, after division by 100, of 7^10 ? [#permalink]

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02 Oct 2014, 01:17
jlgdr wrote:
priyamne wrote:
Ans:

7^10 can be written as 49^5 which can be written as (49^2)^2. 49 when divided by 100 it will give a remainder of (1)^2.49=49 answer (D).

49^5 is not (49^2)^2

Thanks
Cheers!
J

Agree... this approach is incorrect......... $$(49^2)^2 = 49^{(2*2)} = 49^4$$
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Re: What is the remainder, after division by 100, of 7^10 ? [#permalink]

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02 Oct 2014, 01:19
himanshujovi wrote:
Amateur wrote:
what if the answer choise has another value with units digit 9? how do we need to proceed in that case?

Good question. Any clues Bunnuel

When the units digit is 9, the remainder has to be 49. Any other remainder ending with units digit 9 is not possible
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Re: What is the remainder, after division by 100, of 7^10 ? [#permalink]

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10 May 2015, 00:53
Bunuel wrote:
thangvietnam wrote:
if we do not hav formular, how do I do?

What is the remainder, after division by 100, of 7^10 ?

(A) 1
(B) 7
(C) 43
(D) 49
(E) 70

The remainder when 7^10 is divided by 100 will be the last two digits of 7^10 (for example 123 divided by 100 yields the remainder of 23, 345 divided by 100 yields the remainder of 45).

$$7^{10}=(7^2)^5=49^5$$ --> the units digit of 49^5 will be 9 (the units digit of 9^even is 1 and the units digit of 9^odd is 9).

So, we have that $$7^{10}=49^5$$ has the units digit of 9, thus the units digit of the remainder must also be 9. Only answer D fits.

excellent. I can not say a word for this wonderful explanation. thank you Buuney
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Re: What is the remainder, after division by 100, of 7^10 ? [#permalink]

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Re: What is the remainder, after division by 100, of 7^10 ? [#permalink]

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30 May 2016, 08:26
kkalyan wrote:
What is the remainder, after division by 100, of 7^10 ?

(A) 1
(B) 7
(C) 43
(D) 49
(E) 70

7^4 = 2401/100 will have remainder 1
So, 7^8 will have remainder 1

7^10 = 7^8 x 7^2

7^2 will have remainder 49 when divided by 100

So, remainder will be (D) 49

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Re: What is the remainder, after division by 100, of 7^10 ?   [#permalink] 30 May 2016, 08:26
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