I was going through the solutions of various contestants for the problem 1287B. However, I was not able to understand the one that I hereby attach

https://codeforces.com/contest/1287/submission/68285034

There is a statement x[i]^= 66^t[i]; Now, I worked out what it does but , what I cannot get is how 66 was obtained? Is there a mathematical approach being followed or is it just by inspection?