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

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

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New post 16 Sep 2006, 20:02
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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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New post 16 Sep 2006, 20:19
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 Fred´s age today. In y more years will be: F + Y ==>

==> (Y-12)/2 + Y ==> (3/2)Y - 6 ..... (D)
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New post 16 Sep 2006, 20:29
Thank you, I can sleep now...

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New post 17 Sep 2006, 02:56
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

my answer is D

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New post 17 Sep 2006, 04:05
D) 3y/2 - 6

Same working as above

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Re: Math Question driving me nuts!!! Please help [#permalink]

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New post 17 Sep 2006, 11:11
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

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Re: Math Question driving me nuts!!! Please help [#permalink]

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New post 17 Sep 2006, 21:08
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,
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Averages Accelerated:Guide to solve Averages Quickly(with 10 practice problems)


Last edited by cicerone on 25 Sep 2008, 01:25, edited 1 time in total.

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Re: Math Question driving me nuts!!! Please help [#permalink]

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New post 17 Sep 2006, 23:41
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

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New post 19 Sep 2006, 12:43
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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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Re: The sum of Doris and Fred's ages is y . Doris is 12 years [#permalink]

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Re: The sum of Doris and Fred's ages is y . Doris is 12 years   [#permalink] 26 Aug 2017, 01:23
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