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# The sum of Doris and Fred's ages is y . Doris is 12 years

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Intern
Joined: 16 Sep 2006
Posts: 22
The sum of Doris and Fred's ages is y . Doris is 12 years  [#permalink]

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16 Sep 2006, 19:02
The sum of Doris and Fred's ages is y. Doris is 12 years older than Fred. How many years old will Fred be y years from now, in terms of y.
A. y-6
b. 2y-6
C. y/2-6
D. 3y/2-6
E. 5y/2-6

I have the answer by plugging in numbers, but can't figure out how this formula came about. Any help would be greatly appreciated. Thanks.

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Intern
Joined: 23 Jul 2006
Posts: 48

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16 Sep 2006, 19:19
Hi.

You have:

D + F = Y

and you have that:

D = F + 12

If you replace the second equation in the first euation, we have:

2F + 12 = Y => F = (Y-12)/2

That is the FredÂ´s age today. In y more years will be: F + Y ==>

==> (Y-12)/2 + Y ==> (3/2)Y - 6 ..... (D)
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Intern
Joined: 16 Sep 2006
Posts: 22

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16 Sep 2006, 19:29
Thank you, I can sleep now...
Retired Moderator
Joined: 05 Jul 2006
Posts: 1714

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17 Sep 2006, 01:56
The sum of Doris and Fred's ages is y. Doris is 12 years older than Fred. How many years old will Fred be y years from now, in terms of y.
A. y-6
b. 2y-6
C. y/2-6
D. 3y/2-6
E. 5y/2-6

D+F = Y BUT F+12 = D THUS 2F +12 = Y

F NOW = Y-12/2 , F Y YEARS FROM NOW = (Y-12)/2 +Y = 3y/2-6

Senior Manager
Joined: 20 Feb 2006
Posts: 363

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17 Sep 2006, 03:05
D) 3y/2 - 6

Same working as above
Intern
Joined: 17 Apr 2006
Posts: 42

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17 Sep 2006, 10:11
axl169 wrote:
The sum of Doris and Fred's ages is y. Doris is 12 years older than Fred. How many years old will Fred be y years from now, in terms of y.
A. y-6
b. 2y-6
C. y/2-6
D. 3y/2-6
E. 5y/2-6

I have the answer by plugging in numbers, but can't figure out how this formula came about. Any help would be greatly appreciated. Thanks.

Ans: D

Here'z how:

d + f = y, and d = f + 12, (d and f are ages of Doris and Fred)

therefore: 2f + 12 = y or f = (y-12)/2 --- (1)

Fred's age, y years from now is f + 12
= y/2 -6 + y
= 3y/2 - 6
Senior Manager
Joined: 28 Aug 2006
Posts: 296

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Updated on: 25 Sep 2008, 00:25
2
axl169 wrote:
The sum of Doris and Fred's ages is y. Doris is 12 years older than Fred. How many years old will Fred be y years from now, in terms of y.
A. y-6
b. 2y-6
C. y/2-6
D. 3y/2-6
E. 5y/2-6

I have the answer by plugging in numbers, but can't figure out how this formula came about. Any help would be greatly appreciated. Thanks.

Folks, again different ways were given to answer this.
But where is our faster method i.e giving some values and then proceeding...

Let F=24 Clearly D = 36, this mean y=60
After y years ie after 60 years F= 24+60 = 84.

Go to options.........
Placing y=60 Option D gives us 84.
So it is D.........

I recommend all of u to get into these methods

Regards,
_________________

Originally posted by cicerone on 17 Sep 2006, 20:08.
Last edited by cicerone on 25 Sep 2008, 00:25, edited 1 time in total.
Director
Joined: 28 Dec 2005
Posts: 700

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17 Sep 2006, 22:41
axl169 wrote:
The sum of Doris and Fred's ages is y. Doris is 12 years older than Fred. How many years old will Fred be y years from now, in terms of y.
A. y-6
b. 2y-6
C. y/2-6
D. 3y/2-6
E. 5y/2-6

I have the answer by plugging in numbers, but can't figure out how this formula came about. Any help would be greatly appreciated. Thanks.

d+f=y
d=f+2
=>2f+12=y
=>f = (y-12)/2
f+y = (y-12)/2+y = 3y/2 -6

D
Director
Joined: 06 Sep 2006
Posts: 666

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19 Sep 2006, 11:43
1
1
D + F = Y; D = 12 + F; => 12 + F + F = Y => 2F = Y - 12

=> F = (Y - 12)/2

After Y years:
F = (Y - 12)/2 + Y => (Y -12 + 2Y)/2 => (3Y - 12)/2

=> 3Y/2 - 6. So D is my choice.
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Posts: 9836
Re: The sum of Doris and Fred's ages is y . Doris is 12 years  [#permalink]

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13 Oct 2018, 06:47
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Re: The sum of Doris and Fred's ages is y . Doris is 12 years   [#permalink] 13 Oct 2018, 06:47
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