His sister is 10*2= <<10*2=20>>20 years old

Their age difference is 20-10= <<20-10=10>>10 years

Therefore when he is 50 she will be 50+10= <<50+10=60>>60 years old.

# Answer

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<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
John is 10 years old. His sister is twice his age. When he is 50 years old, how old will his sister be?
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
60
John is 10 years old. His sister is twice his age. When he is 50 years old, how old will his sister be?
train