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# If the difference between the squares of the first and last terms of a

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VP
Joined: 07 Dec 2014
Posts: 1224
If the difference between the squares of the first and last terms of a  [#permalink]

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01 Jun 2017, 20:54
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Difficulty:

55% (hard)

Question Stats:

71% (02:53) correct 29% (02:35) wrong based on 132 sessions

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If the difference between the squares of the first and last terms of a sequence of consecutive odd positive integers is 720,
and the first and last terms share the same ones unit, what is the sum of the first and last terms?

A. 64
B. 68
C. 72
D. 76
E. 80
Math Expert
Joined: 02 Sep 2009
Posts: 58435
Re: If the difference between the squares of the first and last terms of a  [#permalink]

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01 Jun 2017, 21:20
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gracie wrote:
If the difference between the squares of the first and last terms of a sequence of consecutive odd positive integers is 720,
and the first and last terms share the the same ones unit, what is the sum of the first and last terms?

A. 64
B. 68
C. 72
D. 76
E. 80

Say the first term is y and the last term is y (notice y > x).

$$y^2 - x^2 = 720$$;

$$(y - x)(y + x) = 2^4*3^2*5$$.

Since both x and y are odd then both y - x and y + x are even.
Since the units digits of x and y are the same then the units digit of y - x is 0.

So, y - x is at least 5*2 = 10 and y + x is at most 3^2*2^3 = 72. In this case y = 41 and x = 31. These two numbers satisfy all the conditions and since a PS question cannot have two correct answer then y + x = 72.

Alternatively for the last step: y - x is at least 5*2 and y + x is at most 3^2*2^3 = 72. From this:
y - x cannot be 5*2*3 = 30 and x = 3*2^3 = 24 because in this case y - x < y + x, which cannot happen.
y - x cannot be 5*2^2 = 20 and x = 3^2*2^2 = 36 because in this case in this case both x and y turn to be even, not odd.

Those were only two other cases, so y - x = 5*2 = 10 and y + x = 3^2*2^3 = 72

Hope it's clear.
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Joined: 27 Mar 2016
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Re: If the difference between the squares of the first and last terms of a  [#permalink]

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01 Jun 2017, 21:31
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Since the number share the same unit digit, difference between the terms would be in multiple of 10.
Let no be x & y => x^2-y^2=720
(x+y)(x-y)=720
(X+y )*10n = 720
X+y=72/n
Since n is an integer and there is no factor of 72 present in the list, answer should be 72.

Sent from my ONEPLUS A3003 using GMAT Club Forum mobile app
##### General Discussion
VP
Joined: 07 Dec 2014
Posts: 1224
If the difference between the squares of the first and last terms of a  [#permalink]

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Updated on: 12 Nov 2018, 12:50
gracie wrote:
If the difference between the squares of the first and last terms of a sequence of consecutive odd positive integers is 720,
and the first and last terms share the same ones unit, what is the sum of the first and last terms?

A. 64
B. 68
C. 72
D. 76
E. 80

if the first and last terms share the same units digit,
then the range of the sequence will be a multiple of 10
because all the answer choices have only two digits, assume the range is 10
let x=the first term; y=the last term
(y+x)(y-x)=720
y-x=range=10
y+x=720/10=72
C

Originally posted by gracie on 01 Jun 2017, 22:10.
Last edited by gracie on 12 Nov 2018, 12:50, edited 1 time in total.
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Joined: 09 Sep 2013
Posts: 13410
Re: If the difference between the squares of the first and last terms of a  [#permalink]

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20 Oct 2018, 09:29
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Re: If the difference between the squares of the first and last terms of a   [#permalink] 20 Oct 2018, 09:29
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