Question 1172287
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<pre>

Let x be the sister current age.

Then  Frank is (x+8) years old.



In 3 years,  the sister age will be (x+3) years,

while Frank will be (x(x+8) + 3) = (x+11) years old.



The condition says that

    x+11 = 2*(x+3).


Simplify and solve

    x + 11 = 2x + 6

    11 - 6 = 2x - x

       5   = x


<U>ANSWER</U>.  The sister is 5 years old.  Frank is 5+8 = 13 years old.
</pre>

Solved.



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The setup equation in the post by @josgarithmetic  &nbsp;&nbsp;{{{highlight((x+8)+3=2(x+8))}}} &nbsp;&nbsp;is &nbsp;TOTALLY &nbsp;WRONG &nbsp;&nbsp;{{{highlight(cross((x+8)+3=2(x+8)))}}} 
and leads to &nbsp;INCORRECT &nbsp;NON-SENSICAL &nbsp;answer &nbsp;x= -5.


For your safety, &nbsp;IGNORE &nbsp;that post.