Question 307124
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Let Jeni be J years old now.
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Glenn is 24 years old.  Glenn is twice as old as Jeni was...
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Stop right there.  Let that "was" be x years ago.  So x years ago, 
Jeni was 12.  Therefore J-x = 12

Also x years ago Glenn was 24-x 

But x years ago was also...  
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...when Glenn was as old as Jeni is now.  
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So Glenn's age then, which was 24-x, is equal to J,
which is Jeni's age now.

So we have this system of two equations in two unknowns:

{{{system(J-x=12,24-x=J)}}}

Solve that and get x=6 and J=18.

So Jeni is 18.

Checking, 6 years ago Glenn was 18, Jeni's age now. And
6 years ago Jeni was 12, and Glenn's age now, 24, is twice
12, Jeni's age then.

Edwin</pre>