Arithmetic sums and accumulation

Develop arithmetic-series reasoning through paired sums, accumulation and missing-parameter problems. Practise choosing between first-term/common-difference and first-term/last-term information, counting actual contributions, and distinguishing an individual term from a running total. Contexts include tile rows and cumulative training distance.

Questions move from direct sums to deriving terms from cumulative formulas, checking whether an exact total is attainable and finding when a target is first reached. Both single-select and multiselect formats include detailed worked explanations and answer checks. Use positive integer term counts, preserve the stated endpoint convention and use exact arithmetic. No geometric-series formulas or induction techniques are required.