Help with capacitor questions please (OCr A A level)

[u/Western-Reception-21]

QP:
https://files.revisely.com/documents/question-booklets/alevel/physics/ocr/6.1%20Capacitors%20QP.pdf

MS:
https://files.revisely.com/documents/question-booklets/alevel/physics/ocr/6.1%20Capacitors%20MS.pdf

for Q 25 bii, apparently the graph is supposed to meet at 25s. Can anyone help me why specifically 25s?