SOLUTION: I am 20 years old and my sister is 8 years old. In how many years will I be twice as old as my sister

Let x = "In how many years", the value under the question.

Then

20 + x = 2*(8+x).


It is your major equation to solve.


20 + x = 16 + 2x  ====>  x = 20 - 16 = 4.


Answer.  In 4 years.


Check.   In 2 years, you will be 24 years old, while your sister will be 8 + 4 12 years old.

         And you will be twice as old as your sister then.