Question 1191750
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Write in symbol: Your age ten years ago if you are twice the age of your sister now. 
Her age now is k years old.
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The sister is k years old now.


You are twice the age of the sister: hence, you are 2k years old.


Ten years ago your age was 2k-10 years.
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Solved.


There is nothing complicated: simply write as you read the problem.