sequence of letters ASSARA...
Source: 2023 South Russian Girls MO - Assara Seniors p8
March 1, 2024
combinatorics
Problem Statement
The girl continues the sequence of letters , adding one of the three letters , or . When adding the next letter, the girl makes sure that no two written sevens of consecutive letters coincide. At some point it turned out that it was impossible to add a new letter according to these rules. What letter could be written last?