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 $30\times2=60$. 
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 \#$$20$$) 

[A] $$40$$ 
[B] $$50$$ 
[C] $$60$$ 
[D] $$70$$ 
[E] $$80$$ 