Common terms of two APs form an AP with difference = LCM

CAT has tested this idea 3 times across 3 papers, from 2019 to 2023. 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 2019 Slot 2 · QA Q28	count common terms of two finite APs	A
CAT 2023 Slot 2 · QA Q22	largest three-digit common term	967
CAT 2023 Slot 3 · QA Q21	sum of common terms for n up to 100	B

Part of Progressions & Series (38 questions since 2017).