Hey, Yeah the answer is 9 because
1/3(9)=12-9
3 = 3
They're equall
First you had to rewrite the problem with eighteen as the three times six is equal to eighteen.

Then you factor it out by the common term of six.

Final answer: 
Hope this helps!
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-Charlie
Let the balance = Y and the number of weeks = x
She takes out 15 per week, so multiply 15 by x ( the number of weeks) to get 15x.
You would then want to subtract that from the amount she started with in her account.
The equation becomes: Y = 150 - 15x