CDS Mathematics — Teaching Notes

Number System is the single biggest chapter in CDS Elementary Mathematics: 218 past-year questions across all twenty sittings from 2016 (II) to 2026 (I), which is roughly eleven of the hundred questions on every paper. Nothing else in the syllabus pays that well. The paper gives you 100 questions in 120 minutes at plus one and minus one-third, so the job here is not depth but SPEED with certainty — almost every question below is a thirty-second question once you recognise the shape, and a three-minute question if you do not. The notes are sequenced as a teaching arc, not as a filter list: the primitives first (division, parity, consecutive integers), then how a number is WRITTEN (place value, divisibility rules, unit digits), then how it is BUILT (primes, divisors, HCF and LCM), then the hard end where remainders and factorisation live. Difficulty follows that order on its own — the two densest HARD units in the chapter sit at positions nine and ten. Work them in order.

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