Donot say that As from posting, you have already shared some important info with us: (1) It is an oral interview; (2) 90 - 50 = 40 min to prepare for the answers; (3) 50 min for interview.
Thanks for the sharing.
No need to worry 15 min, one of my colleague only took 5 min to pass the interview. Sometimes, hiring decision can even be made in 1 min.