4. How old is each of us? I am now twice as old as you were when I was as old as you are now. Together, we are 35 years old. How old is each of us?
4. Let $2x$ be my current age, and $y$ be your current age, then $y$ was my age back then, and $x$ was your age back then. How many years have passed? We can calculate it in two ways: $2x - y = y - x$. Moreover, it is known that $2x + y = 35$. From the first equation, we get: $2y = 3x$. On the other hand, $y = 35 - 2x$. Therefore: $70 - 4x = 3x$ and $x = 10; y = 15$. So, I am currently 20 years old, and you are 15.