Let the original solution contain \( x \) liters of water and \( y \) liters of acid.
When 2 liters of water are added, the new volume of water is \( x + 2 \) liters, with the acid volume remaining \( y \) liters.
This new solution is 50% acid, which can be expressed as:
\[ \frac{y}{x + 2} = 0.5 \]
Next, 15 liters of acid are added. The solution now has:
Water: \( x + 2 \), Acid: \( y + 15 \)
This final solution is 80% acid, leading to the equation:
\[ \frac{y + 15}{x + 2 + 15} = 0.8 \]
Solving these two equations yields:
\[ x = 2, \quad y = 7 \]
Thus, the original solution's water to acid ratio is \( 2 : 7 \), or equivalently \( 1 : 3.5 \).