Capacitor charging/discharging waveform confusion
The 22.5ms label is wrong, but the rest of the graph is right. There's equal horizontal spacing between 0ms, 5ms, 10ms, and 15ms, with about half as much space between 15ms and the final +5τ' tick. This suggests that the +5τ' tick is actually at 17.5ms, which is the correct answer.

Textbooks sometimes have errata on the publisher's web site. You can check there to see if they've already noticed this error. If not, they should have a way for you to report it.