Mudrat,
You have to do your 'working' for questions like that out loud in the interview - they care about the process, not the correct answer.
So for me, 235000/12 would be:
235000/10 = 23500, so divide by 12 would be about 20000.
OR 240000/12 = 20000, so 235000 must be a bit less than that, say 19500-ish?
Seemed to work for me (long ago!!)