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Joined: 07 Dec 2014
Posts: 1221
If the difference between the squares of the first and last terms of a  [#permalink]

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Difficulty:   55% (hard)

Question Stats: 71% (02:50) correct 29% (02:35) wrong based on 133 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 V
Joined: 02 Sep 2009
Posts: 59199
Re: If the difference between the squares of the first and last terms of a  [#permalink]

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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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GMAT 1: 590 Q44 V22 Re: If the difference between the squares of the first and last terms of a  [#permalink]

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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.

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##### General Discussion
VP  P
Joined: 07 Dec 2014
Posts: 1221
If the difference between the squares of the first and last terms of a  [#permalink]

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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 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.
Non-Human User Joined: 09 Sep 2013
Posts: 13624
Re: If the difference between the squares of the first and last terms of a  [#permalink]

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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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