1. The problem is to write the ordinal number for the cardinal number thirty-one. 2. Ordinal numbers indicate position or order in a sequence, such as first, second, third, etc. 3. The cardinal number 31 corresponds to the ordinal number "thirty-first". 4. Ordinal numbers for numbers ending in 1 (except 11) typically end with "-st". 5. Therefore, the ordinal number for 31 is written as **thirty-first**.