Errand Chronicle 31011 · Entry #8

Pyresh's Formula VIII

Objective
Found a trace of Pyresh. Let's solve Pyresh's Formula written on it.
Solve the secret formula left behind by Pyresh and type it in the chat window;
Dialogue
Trace of Pyresh Dialogue
Pyresh left behind a piece of his realizations.
Numbers are written in what looks to be a riddle of sorts.
After a sequence of numbers follows a question mark.
It's as if someone wanted their formula to be solved by another.
?(100) = 97
?(90) = 89
?(80) = 79
?(70) = ?
Trace of Pyresh Dialogue (Alt)
?(100) = 97
?(90) = 89
?(80) = 79
?(70) = ?
Trace of Pyresh Completion
The answer was 67.
This problem involved finding the largest prime number
smaller than the number in the parenthesis.
You looked for a number only divisible by 1 and itself.
Your head is aching and feel dizzy,
yet you're becoming more curious about Pyresh's whereabouts.
Other Errands in This Chronicle

9 further errands share this chronicle — the same travelers, the same stretch of road.

Details
Chronicle31011
Entry8
Quest Giver
Turn-in
Summary
The answer was 67.
This problem involved finding the largest prime number
smaller than the number in the parenthesis.
You looked for a number only divisible by 1 and itself.
Your head is aching and feel dizzy,
yet you're becoming more curious about Pyresh's whereabouts.