7. Xiao Dong is 8 years younger than his sister. In 3 years, his sister's age will be twice Xiao Dong's age. His sister is $\qquad$ years old this year.
【Analysis】Solution 1: 13 years old, let the sister be $x$ years old this year, and Xiao Dong be $x-8$ years old this year, $x+3=2(x-5)$, solving for $x$ gives $x=13$ Solution 2: The age difference remains constant, in 3 years the sister will be 16 years old, and Xiao Dong will be 8 years old; so the sister is 13 years old now.