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# If n is a positive integer, how many of the ten digits from

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If n is a positive integer, how many of the ten digits from [#permalink]

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08 Feb 2014, 10:52
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54% (01:58) correct 46% (01:02) wrong based on 286 sessions

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If n is a positive integer, how many of the ten digits from 0 through 9 could be the units digits of n^3 ?

A. Three
B. Four
C. Six
D. Nine
E. Ten

Can anyone help me with this question? Thanks!
[Reveal] Spoiler: OA
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Re: If n is a positive integer, how many of the ten digits from [#permalink]

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08 Feb 2014, 12:43
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Im also not completely sure about this one. But if you simply plug in values for ''n'' from 0 tot 9.
This will show you that units digit can be 0,1,2,3,4,5,6,7,8,9 = 10.
0^3=0 , 1^3=1 , 2^3=8 , 3^3=27, 4^3=64, 5^3=125, 6^3=216, 7^3=343, 8^3=512, 9^3=729

These possibilities keep repeating themselves.
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Last edited by CarloCjm on 09 Feb 2014, 03:15, edited 1 time in total.
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Re: If n is a positive integer, how many of the ten digits from [#permalink]

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08 Feb 2014, 13:06
I did it exactly like carlocjm explained, in around 2 minutes. Keep in mind that you don't have to go through the whole multiplication process, just get the units digits and stop, since that's what we're concerned about. Also, for some numbers, you actually don't have to do manual stuff (5^3 is by heart 125, for example).

I'd say (E), too. Waiting for other solutions.
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Re: If n is a positive integer, how many of the ten digits from [#permalink]

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02 Apr 2014, 08:18
Any one with a better solution for this?
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Re: If n is a positive integer, how many of the ten digits from [#permalink]

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02 Apr 2014, 08:31
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Qoofi wrote:
Any one with a better solution for this?

If n is a positive integer, how many of the ten digits from 0 through 9 could be the units digits of n^3 ?

A. Three
B. Four
C. Six
D. Nine
E. Ten

The question is fairly straightforward, it basically asks: which digits can be the units digit of a perfect cube.

Can it be 0? Yes, 10^3=1,000.
Can it be 1? Yes, 1^3=1.
Can it be 2? Yes, 8^3=512.
Can it be 3? Yes, 7^3=343.
Can it be 4? Yes, 4^3=64.
Can it be 5? Yes, 5^3=125.
Can it be 6? Yes, 6^3=...6.
Can it be 7? Yes, 3^3=27.
Can it be 8? Yes, 2^3=8.
Can it be 9? Yes, 9^3=...9.

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Re: If n is a positive integer, how many of the ten digits from [#permalink]

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03 Apr 2014, 02:23
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For a number to be a cube, any digit from 0 to 9 may be in the units place,

however for a number to be a square, there are only six possibilities in units place which are

0, 1, 4, 5, 6, 9
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Re: If n is a positive integer, how many of the ten digits from [#permalink]

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03 Apr 2014, 12:36
I'm confused. I got D) 9, because I read "n is a positive integer" and immediately wrote down on my paper n>0. So I did not consider 0^3.

I thought zero was not positive or or negative. Can someone confirm??
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Re: If n is a positive integer, how many of the ten digits from [#permalink]

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03 Apr 2014, 12:43
macey15 wrote:
I'm confused. I got D) 9, because I read "n is a positive integer" and immediately wrote down on my paper n>0. So I did not consider 0^3.

I thought zero was not positive or or negative. Can someone confirm??

Yes, zero is neither positive nor negative.

But 0 still can be the units digit of a perfect cube, consider 10^3=1,000.
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Re: If n is a positive integer, how many of the ten digits from [#permalink]

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03 Apr 2014, 13:37
Bunuel wrote:
macey15 wrote:
I'm confused. I got D) 9, because I read "n is a positive integer" and immediately wrote down on my paper n>0. So I did not consider 0^3.

I thought zero was not positive or or negative. Can someone confirm??

Yes, zero is neither positive nor negative.

But 0 still can be the units digit of a perfect cube, consider 10^3=1,000.

Of course!! For some reason I got stuck on n = 1-9, which looking back, the problem definitely does NOT say. Thanks!
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Re: If n is a positive integer, how many of the ten digits from [#permalink]

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Re: If n is a positive integer, how many of the ten digits from [#permalink]

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02 Oct 2015, 14:44
CarloCjm wrote:
Im also not completely sure about this one. But if you simply plug in values for ''n'' from 0 tot 9.
This will show you that units digit can be 0,1,2,3,4,5,6,7,8,9 = 10.
0^3=0 , 1^3=1 , 2^3=8 , 3^3=27, 4^3=64, 5^3=125, 6^3=216, 7^3=343, 8^3=512, 9^3=729

These possibilities keep repeating themselves.

You got to the right conclusion, but be careful: the question states n has to be a POSITIVE integer, therefore 0 is excluded. The 0 units digit comes from 10^3
Re: If n is a positive integer, how many of the ten digits from   [#permalink] 02 Oct 2015, 14:44
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