In English,  "twice older" = "three times as old".



Is it what you mean ?  - If so, then Mary is 3*(11-5) + 5 = 3*6 + 5 = 18 + 5 = 23 years old now.

If 5 years ago, Mary WAS TWICE OLDER than her sister, then Mary is now 23.

If 5 years ago, Mary WAS TWICE AS OLD AS her sister , then Mary is now 17.

You figure it out since the problem, like about 90% of these age problems in this forum, is FAULTY. 

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