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Anna is $$10$$ years old and the age of her mother is $$4$$ times that of Anna. How old will Anna\textquotesingle s mother be when Anna\textquotesingle s mother is twice as old as Anna? (Adapted from $$2007$$ Math kangaroo Problem, Level $$5-6$$, Question \#$$20$$) 
Overseas Competition->Knowledge Point->Word Problem Modules->Age Problems->Varying Multiples in Age Problems
The age difference between them is $(4-1)\times10=30$, which stays the same forever. Then, when Anna\textquotesingle s mother is twice as old as Anna, Anna should be $30$ years old, then her mom should be $30\times2=60$. 