Let's call the age of the third person Y.
According to the problem, the present ages of the three people are 13, 7, and X.
X years ago, their ages were X+13, X+7, and Y.
We know that the product of their ages X years ago was 55:
(X+13)(X+7)(Y) = 55
We can simplify this expression by expanding the left side:
XY + 20X + 91Y + 91 = 55
Subtracting 91 from both sides and rearranging, we get:
XY + 20X + 91Y - 36 = 0
This equation is not solvable without additional information.