I can't figure it out!
THis is a great problem, but maybe not for the students!
Centripetal force must be provided by some other recognizable force. In this case, it is provided by the horizontal componant of the tension.
The vertical componant of the tension has to balance the only other force on the chair (what could it be?).
You have the angle of the cable, and you can calculate the weight of the chair. Can you take it from there?
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