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Re: If x is a positive integer, what is the units digit of 3x? [#permalink]

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26 Mar 2012, 18:32

in stmt 1, no matter what variable k is since it is greater than 0 and an integer 10k^2+1 will always be 1 and so 1*3 will be 3.... sufficient stmt 2 = 11 fits the equation and also 9 fits the equation. so it is insufficient what the unit variable is.
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Re: If x is a positive integer, what is the units digit of 3x? [#permalink]

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Re: If x is a positive integer, what is the units digit of 3x? [#permalink]

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24 Nov 2016, 04:13

Nice Question. Here is what i did => We need the units digit of 3x so we need the unit digit of x and we will multiply it by 3 to get to the units digit of 3x

Statement 1 here since k>0 and an integer. The units digit of x must be 1 hence the units digit of 3x must be 3 hence sufficient Statement 2 the units digit of x^2 is 1 so the units digit of x can be 1 or 9 hence the units digit of 3x can be 3 or 7

Re: If x is a positive integer, what is the units digit of 3x? [#permalink]

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29 May 2017, 10:21

siddhans wrote:

If x is a positive integer, what is the units digit of 3x?

(1) x = 10k^2 + 1, where k is a positive integer. (2) The units digit of x^2 is 1.

This problem can be quickly solved via creating a table. Yes, you can reason your way through it, but it drains mental energy. The goal is to find the UNIQUE value of the units digit of 3x. This number ranges from 0 to 9, since it's a place value.

Statement 1) x = 10k^2 + 1

Create a table here with columns corresponding to k, 10k^2 +1 = x, and 3x

Only two x values have x^2 values with a 1 in the units digit, so that's good, but unfortunately the units value of 3x for those two x values differ (3 and 7, respectively). Insufficient.

Re: If x is a positive integer, what is the units digit of 3x? [#permalink]

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25 Sep 2017, 06:22

siddhans wrote:

If x is a positive integer, what is the units digit of 3x?

(1) x = 10k^2 + 1, where k is a positive integer. (2) The units digit of x^2 is 1.

St 1

This statement basically tells us that the units digit of X will always be 1 because no matter what K is, and K must be a positive integer, 10k^2 will always be a multiple of 10 and thus have a units digit of 0. If this is true then x will always have a units digit of 1 and if x always has a units digit of 1 then the units digit of 3x will always be 3