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Let "x" be the girl's age.
The condition says
x = 2*(x-9).
====> x = 2x - 18 ====> x = 18.
Solved.
There is a bunch of lessons on age word problems in this site:
- Age problems and their solutions
- A fresh formulation of a traditional age problem
- Really intricate age word problem
- Selected age word problems from the archive
Read them and become an expert in solving age problems.
Also, you have this free of charge online textbook in ALGEBRA-I in this site
- ALGEBRA-I - YOUR ONLINE TEXTBOOK.
The referred lessons are the part of this online textbook under the topic "Age word problems".