The first term of an arithmetic progression is -20 and the common difference is 5.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2024). The mark or mark type is shown in brackets.

1. [M1] Correct use of either S20 formula with a = -20 and d = 5.
2. [A1] 550
3. [M1*] Correct use of Sn formula with a=−20, d=5 and either k or 2k. This mark can be awarded for clear use of n/2 (a+l) when correct values of a and d are used.
4. [DM1] Equating their S2k to 10 × their Sk and reaching a 2-term quadratic or 2 term linear equation if k has been cancelled. Condone errors in simplification.
5. [A1] k=12 Condone extra solution k = 0.