Code Equation

Exploration Stage: Post Office
From: Kira Woodville
Description: The inventor's suitcase is filled with some kind of glass covered with formulas. I sorted them by number for an hour to put in the file! Hey, where did tablet go? Black will skin me alive! Please help me find it!
Completed: This is the one. Without your help, I wouldn't have found this glass piece full of formulas! And why did the inventor write his formulas on transparent glass? Has he never heard of paper?