Ratio of Water and Acid in Solution

Let the original solution have $A$ litres of acid and $W$ litres of water.

Step 1: Adding 2 litres of water makes acid concentration 50%: $A = W + 2$

Step 2: Adding 15 litres of acid makes acid concentration 80%: $\frac{A + 15}{(A + 15) + (W + 2)} = \frac{4}{5} \implies A + 15 = 4(W + 2)$

Substitute $A = W + 2$: $(W + 2) + 15 = 4W + 8 \implies 3W = 9 \implies W = 3$ Then $A = 3 + 2 = 5$.

Ratio of water to acid in the original solution $= W : A = 3 : 5$.