Solving Connections: Joe is 5 years older than Sally. In 15  TOPIC_SOLVED

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Solving Connections: Joe is 5 years older than Sally. In 15

Postby Xo Exia oX on Tue Jul 28, 2009 6:40 pm

Joe is 5 years older than Sally. In 15 years Joe will be 3 times as old as Sally is now. How old is Sally now? Hint: Write Joe's age now in terms of Sally's age now. Then write Joe's age in 15 years using the expression you just wrote for Joe's age now. Then using S for Sally's age now, write the equation with only one variable "S" and solve it.


I tried to solve for S, and I got a negative number. I was confused it turned out to be negative. Help?
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Postby stapel_eliz on Tue Jul 28, 2009 7:18 pm

Please reply showing your steps. Once we can see what you've done, we'll have better luck finding where you may have gone wrong.

Thank you! :D
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Re: Solving Connections: Joe is 5 years older than Sally. In 15

Postby Xo Exia oX on Wed Jul 29, 2009 12:59 pm

S + 5 = S

3(S + 5) = S

3S + 15 = S
- 15 -15

3S = S - 15
-S -S

2S = -15
/2 /2

S = -7.5
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Postby stapel_eliz on Wed Jul 29, 2009 2:56 pm

Xo Exia oX wrote:S + 5 = S

For what does S stand?

Note: The above equation has no solution, so there's something wrong with the logic of it. How did you arrive that this equation?

Try studying the general methodology of this sort of exercise here. Then, following the logic explained there (and in what was given to you within the exercise itself):

Joe is defined in terms of Sally, so:

i) Pick a variable to stand for Sally's age now.

ii) Write an expression, in terms of the variable in (i), for Joe's age now.

iii) Write an expression, in terms of the variable in (i), for Sally's age after she's fifteen years older.

iv) Write an expression, in terms of the expression in (ii), for Joe's age after he's fifteen years older.

v) Using the expression in (iii) and the variable in (i), write an equation for "(Joe's age after fifteen years) is (three) times (Sally'es age now)".

vi) Solve the equation in (v).

vii) Use the definition in (i) to interpret the answer to (vi).

If you get stuck, please reply showing your work for each of the steps above, as far as you were able to get.

Thank you! :D
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Re: Solving Connections: Joe is 5 years older than Sally. In 15

Postby Xo Exia oX on Thu Jul 30, 2009 5:08 pm

1)S

2)S + 5

3) S + 15

4) S + 20

5) S + 20 = 3S
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  TOPIC_SOLVED

Postby stapel_eliz on Thu Jul 30, 2009 5:22 pm

Very good! Now solve the linear equation:

Subtract "S" from either side.

Divide through by "2".

:wink:
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