What rank did Belshazzar give Daniel for reading the writing on the wall?

Answer: Third ruler in the kingdom

Belshazzar had Daniel clothed in purple, given a gold chain, and proclaimed the third highest ruler in the kingdom.

“But I have heard about you, that you are able to give interpretations and solve difficult problems. Therefore, if you can read this inscription and give me its interpretation, you will be clothed in purple and have a gold chain placed around your neck, and you will be made the third highest ruler in the kingdom.””— Daniel 5:16 (BSB)