If t is a positive integer and r is the remainder when t^2+5 : GMAT Data Sufficiency (DS)
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# If t is a positive integer and r is the remainder when t^2+5

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If t is a positive integer and r is the remainder when t^2+5 [#permalink]

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23 Feb 2012, 14:21
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If t is a positive integer and r is the remainder when t^2+5t+6 is divided by 7, what is the value of r?

(1) When t is divided by 7, the remainder is 6.
(2) When t^2 is divided by 7, the remainder is 1.

[Reveal] Spoiler:
...
(1) means t=7n+6.
=> t²=(7n+6)²)=49n²+84n+36=49n²+84n+35+1
=> 5t=35n+30=35n+28+2

remainders 1+2+6=9=7+2

Remainder of t²+5t+6 is 2????

I dont really get this -.-
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If t is a positive integer and r is the remainder when t^2+5 [#permalink]

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23 Feb 2012, 14:50
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If t is a positive integer and r is the remainder when t^2+5t+6 is divided by 7, what is the value of r?

First of all factor $$t^2+5t+6$$ --> $$t^2+5t+6=(t+2)(t+3)$$.

(1) When t is divided by 7, the remainder is 6 --> $$t=7q+6$$ --> $$(t+2)(t+3)=(7q+8)(7q+9)$$. Now, no need to expand and multiply all the terms, just notice that when we expand all terms but the last one, which will be 8*9=72, will have 7 as a factor and 72 yields the remainder of 2 upon division by 7. Sufficient.

(2) When t^2 is divided by 7, the remainder is 1 --> different values of t possible: for example t=1 or t=6, which when substituted in $$(t+2)(t+3)$$ will give different remainder upon division by 7. Not sufficient.

Hope it's clear.
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Re: Tricky? remainder question (gmatprep) [#permalink]

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05 Jul 2012, 10:18
Bunuel wrote:
If t is a positive integer and r is the remainder when t^2+5t+6 is divided by 7, what is the value of r?

First of all factor t^2+5t+6 --> t^2+5t+6=(t+2)(t+3)

(1) When t is divided by 7, the remainder is 6 --> t=7q+6 --> (t+2)(t+3)=(7q+8)(7q+9). Now, no need to expand and multiply all the terms, just notice that when we expand all terms but the last one, which will be 8*9=72, will have 7 as a factor and 72 yields the remainder of 2 upon division by 7. Sufficient.

(2) When t^2 is divided by 7, the remainder is 1 --> different values of t possible: for example t=1 or t=6, which when substituted in (t+2)(t+3) will yield different remainder upon division by 7. Not sufficient.

Hope it's clear.

Sorry Bunuel but this part highlighted is not completely clear: on the left side of = we have the two roots of our equation; on the right (t+2) ----> is 7q+6+2 ---> 7q+8 (same reasoning for 7q+9) BUT why we equal this two part ?? Can you explain me please ??

Thank you
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Re: Tricky? remainder question (gmatprep) [#permalink]

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05 Jul 2012, 10:28
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carcass wrote:
Bunuel wrote:
If t is a positive integer and r is the remainder when t^2+5t+6 is divided by 7, what is the value of r?

First of all factor t^2+5t+6 --> t^2+5t+6=(t+2)(t+3)

(1) When t is divided by 7, the remainder is 6 --> t=7q+6 --> (t+2)(t+3)=(7q+8)(7q+9). Now, no need to expand and multiply all the terms, just notice that when we expand all terms but the last one, which will be 8*9=72, will have 7 as a factor and 72 yields the remainder of 2 upon division by 7. Sufficient.

(2) When t^2 is divided by 7, the remainder is 1 --> different values of t possible: for example t=1 or t=6, which when substituted in (t+2)(t+3) will yield different remainder upon division by 7. Not sufficient.

Hope it's clear.

Sorry Bunuel but this part highlighted is not completely clear: on the left side of = we have the two roots of our equation; on the right (t+2) ----> is 7q+6+2 ---> 7q+8 (same reasoning for 7q+9) BUT why we equal this two part ?? Can you explain me please ??

Thank you

We are asked to find the remainder when $$t^2+5t+6$$ is divided by 7 or as $$t^2+5t+6=(t+2)(t+3)$$, the remainder when $$(t+2)(t+3)$$ is divided by 7.

Now, from (1) we have that $$t=7q+6$$. Substitute $$t$$ with $$7q+6$$ in $$(t+2)(t+3)$$ to get $$(7q+8)(7q+9)$$. So, finally we have that we need to find the remainder when $$(7q+8)(7q+9)$$ is divided by 7 ($$t^2+5t+6=(t+2)(t+3)=(7q+8)(7q+9)$$).

Hope it's clear.
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Re: If t is a positive integer and r is the remainder when t^2+5 [#permalink]

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05 Jul 2012, 14:58
Thanks Mod now is clear .
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Re: If t is a positive integer and r is the remainder when t^2+5 [#permalink]

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08 Feb 2013, 09:37
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My approach:

From (1) - t/7 gives remainder = 6. So we can easily find remainder for t^2 and 5*t in the algebraic exp. t^2 + 5t + 6 using properties of remainder.

Therefore the remainder of algebraic exp. = 1 + 2 + 6 = 9 (7 + 2) = 2 ................. [A]

Properties of remainder:

[1] If a quotient is squared, the remainder also gets squared and we need to adjust the new remainder based on no. we get on squaring.

Here quotient = t, r = 6. So, t^2 will have r' = 36 (7*5 + 1). Therefore r' =1

[2] If a quotient is multiplied by an integer k , the remainder also gets multiplied and we need to adjust the new remainder based on no. we get on multiplication.

Here quotient = t, r = 6. So, 5t will have r'' = 30 (7*4 + 2). Therefore r'' = 2

We get [A] using [1] and [2].

Clearly Stmt (2) can have multiple possibilities i.e. if t= 1 (t^2 =1) and t =6 (t^2 = 36) both give remainder 1, so no unique answer and hence rejected.

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Re: If t is a positive integer and r is the remainder when t^2+5 [#permalink]

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06 Nov 2013, 08:34
Wow. This is a tough problem. A is straightforward once you see it. B is time consuming if you take it all the way.
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If t is a positive integer and r is the remainder when t^2+5 [#permalink]

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25 Aug 2014, 11:51
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(2) When t^2 is divided by 7, the remainder is 1 --> different values of t possible: for example t=1 or t=6, which when substituted in (t+2)(t+3) will give different remainder upon division by 7. Not sufficient.

Bunuel could you please explain why statement 2 is not sufficient and how the values for 't' could be either 1 or 6?
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Re: If t is a positive integer and r is the remainder when t^2+5 [#permalink]

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06 Sep 2014, 04:17
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Shehryar Khan wrote:
(2) When t^2 is divided by 7, the remainder is 1 --> different values of t possible: for example t=1 or t=6, which when substituted in (t+2)(t+3) will give different remainder upon division by 7. Not sufficient.

Bunuel could you please explain why statement 2 is not sufficient and how the values for 't' could be either 1 or 6?

Hii Shehryar khan,
let me try to explain..

t^2/7=1----------given

as u can see,we have 2 values of 2..1^2/7=remainder 1 and 6^2/7=36/7=remainder 1..
when we substitute 1 in (t+2)(t+3)/7 ,we get remainder as 5..
when we substitute 6 in (t+2)(t+3)/7, we get remainder as 2..

So,since the results are inconsistent,we cant have one definite remainder,which we are required to find,as per the stem..so,not sufficient..

Please consider KUDOS if my post helped
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Re: If t is a positive integer and r is the remainder when t^2+5 [#permalink]

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Re: If t is a positive integer and r is the remainder when t^2+5 [#permalink]

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14 Oct 2016, 14:13
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If t is a positive integer and r is the remainder when t^2+5 [#permalink]

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15 Oct 2016, 06:31
dpark wrote:
If t is a positive integer and r is the remainder when t^2+5t+6 is divided by 7, what is the value of r?

(1) When t is divided by 7, the remainder is 6.
(2) When t^2 is divided by 7, the remainder is 1.

FROM STATEMENT - I INSUFFICIENT

The minimum value of t = 13

So, $$t^2+5t+6 = 13^2+5*13+6$$ = $$240$$

Now, $$\frac{240}{7}$$ = Remainder 2

FROM STATEMENT - II INSUFFICIENT

The minimum value of t = +1 and -1

So, $$t^2+5t+6 = 1^2+5*1+6$$ = $$12$$ Remainder 5

And $$t^2+5t+6 = -1^2+5*-1+6$$ = $$2$$ Remainder 2

Thus, Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient to answer the question asked, hence correct answer will be (A)

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If t is a positive integer and r is the remainder when t^2+5 [#permalink]

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04 Jan 2017, 03:14
I am sure that I am missing something because stem (1) tells that t=7q+6 so t=6,13,20,...
then we can try different values of t so that we can find out the reminder,
so it follows that (t+2)(t+3)=5p+r
for t=13 (13+2)(13+3)=240, 240/5 gives a reminder of 0
for t=20 (20+2)(20+3)=506, 506/5 gives a reminder of 1.
So stem (1) Not sufficient...
Please, can someone explain to me what I am missing? Thanks
If t is a positive integer and r is the remainder when t^2+5   [#permalink] 04 Jan 2017, 03:14
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