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# The sum of the digits of 10^x−1 is equal to 3^8. What is the value of

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Math Expert
Joined: 02 Sep 2009
Posts: 43804
The sum of the digits of 10^x−1 is equal to 3^8. What is the value of [#permalink]

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10 Oct 2017, 20:32
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Question Stats:

54% (01:17) correct 46% (01:41) wrong based on 122 sessions

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The sum of the digits of $$10^x−1$$ is equal to 3^8. What is the value of x?

A. 18
B. 243
C. 729
D. 2187
E. 6561
[Reveal] Spoiler: OA

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Manager
Joined: 12 Feb 2017
Posts: 71
Re: The sum of the digits of 10^x−1 is equal to 3^8. What is the value of [#permalink]

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10 Oct 2017, 21:20
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lets generalize this sum,
consider x=1,
10^1 - 1= 9 ------9--- 3^2
x=2,
10^2 - 1=99 ----18---2*3^2
x=3,
10^3 - 1=999----27---3*3^2
x=4,
10^4 - 1=9999---36---4*3^2

similarly,
10^x-1= 999...x times= x*9---- x*3^2
It is given that,
x*3^2 = 3^8
hence, x=3^6
x=729

Answer is option C.

Kudos if it helps.
Intern
Joined: 30 Jul 2017
Posts: 24
Re: The sum of the digits of 10^x−1 is equal to 3^8. What is the value of [#permalink]

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20 Oct 2017, 11:18
Could you please clarify how to calculate this in a bit more comprehensive way?
Intern
Joined: 03 Aug 2017
Posts: 19
Location: United States (NY)
GMAT 1: 720 Q49 V39
GPA: 3.76
Re: The sum of the digits of 10^x−1 is equal to 3^8. What is the value of [#permalink]

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20 Oct 2017, 16:10
krikre wrote:
Could you please clarify how to calculate this in a bit more comprehensive way?

10^x - 1 = 3^8

It is worth noting when starting this problem that 10-1 = 9, 10^2 - 1 = 99, etc. So whatever X equals, is how many 9s that number will have.

Also:
3^8 = (3^2)^4 = 9^4

So to find out what X is we need to determine how many 9's are in 9^4

Well, (9) (9^1) has only one 9
(9)*9 (9^2) has nine, 9s
(9)*9*9 (9^3) has eighty-one 9s
and (9)*9*9*9 (9^4) has 729 9s.

Thus 10^729 - 1 will have the same amount of 9's as 9^4
Math Expert
Joined: 02 Aug 2009
Posts: 5649
Re: The sum of the digits of 10^x−1 is equal to 3^8. What is the value of [#permalink]

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20 Oct 2017, 19:36
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Bunuel wrote:
The sum of the digits of 10^x−1 is equal to 3^8. What is the value of x?

A. 18
B. 243
C. 729
D. 2187
E. 6561

$$10^x-1$$ will always lead to a number with digits as 9s...
so $$10^x-1 = 99999...x$$ times
sum of digits =$$9+9+9+...x$$ times = $$x*9$$

so $$x*9=3^8=3^6*9.......x=3^6=729$$

C
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Re: The sum of the digits of 10^x−1 is equal to 3^8. What is the value of   [#permalink] 20 Oct 2017, 19:36
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# The sum of the digits of 10^x−1 is equal to 3^8. What is the value of

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