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Karl is now $10$ years old, and Ala is $3$ years old. How many years from now will Karl be twice as old as Ala? 
Overseas Competition->Knowledge Point->Word Problem Modules->Age Problems
The age difference is equal to $10-3=7$. When the age of Karl is twice the age of Ala, the age of Ala will be equal to the age difference which is $7$. Thus, it will pass $7-3 = 4$ years. 