Answer:
Tanya is 14; Ruby is 11. 
Explanation:
Let T represent Tanya's current age and let R represent Ruby's current age. 
We know that their ages add up to 25. So: 

8 years ago, Tanya was twice as old as Ruby. In other words, Tanya's current age minus 8 is the same as Ruby's current age minus 8 times 2. So: 

We have a system of equations. We can solve by substitution. From the first equation, subtract R from both sides: 

Substitute this into the second equation: 

On the left, subtract. On the right, distribute: 

Add 16 to both sides. The right side cancels: 

Add R to both sides. The left cancels: 

Divide both sides by 3: 

So, Ruby is currently 11 years old. 
So, Tanya is currently 25-11 or 14 years old. 
Check:
8 years ago, Ruby was 11-8 or 3 years old. 
8 years ago, Tanya is 14-8 or 6 years old. 
Tanya's age of 6 is 2 times Ruby's age of 3 so our answer is correct.