I’m not the only person who has an issue with this practice problem. Here is a link to the problem.
The issue is: b can be an infinite sequence. I have no problem calculating b to an arbitrary length. In the example given for the problem, b is given to the 16th digit. Why the 16th? Given an unknown input sequence, how can we know if an arbitrary sequence can be found or not? It can be proven that some sequences of c will not appear, but how can we know when to stop looking?