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