Anthony is 5 years older than his sister ellen, and 7 years ago the sum of their ages was 23 years. in how many years will anthony be 30 years old?

Now:                         7 years ago: 
Anthony = x + 5                Anthony = x + 5 - 7
Ellen = x                             Ellen = x - 7 
                               
                                  x - 2 + x - 7 = 23
                                     2x -9 = 23 
                                    2x = 32
                                    x = 16

Now Anthony's age = x + 5 = 16 + 5 = 21

30 - 21 = 9 

In 9 years Anthony will be 30 years old.