Repeated replacement of a mixture: concentration × (1 − f) each time

CAT has tested this idea 6 times across 6 papers, from 2018 to 2025. Solve one, and the others are the same method with different numbers or wording, which is why repeated concepts are the most efficient thing to practise.

Question	What it asks	Answer
CAT 2024 Slot 1 · QA Q17	two-thirds replaced with water four times; final milk:water	D
CAT 2023 Slot 2 · QA Q14	4 of 40 litres replaced; count steps until milk below half	7
CAT 2021 Slot 2 · QA Q4	9 litres replaced twice giving 16:9; find capacity	45
CAT 2023 Slot 1 · QA Q11	coffee replaced by cocoa twice, 16:9; compare cocoa in P and Q	C
CAT 2018 Slot 2 · QA Q19	10% replaced twice, starting mixture already has water	D
CAT 2025 Slot 1 · QA Q9	three steps, alternating water and acid replacements	D

Part of Ratio, Mixtures & Averages (84 questions since 2017).