Ben is now 15.
Jerry is now 17.
You can solve this by logic.
In 3 years, the sum of their ages will be 38.
Pretend just for a minute that they are the same age as each other.
That means that in 3 years they would be 19 years old each.
But they're not the same age.
They're two years apart.
So tweak the numbers to make one number 2 years more than the other.
Subtract one year from one of the 19s and add it to the other 19.
That means that three years from now they will actually be 18 and 20.
So now they are 15 and 17.