TxO Math Bowl 2024 - Guts Contest - Set 1 Problem 2

Anna is performing some scientific experiments. For reasons unknown to mankind, she has 100 litres of water. She starts by replacing 20 litres of water with pure vinegar. Next, Anna replaces 25 litres of the new mixture with pure water. If the ratio of vinegar to water in the final mixture is \frac{a}{b}, where a and b are relatively prime, positive integers, find a + b.

(Assume that the vinegar is uniformly spread across the mixture).