Meta Data Scientist interview questions

Two tables: User actions: date|sessio_id|userid|event session details: date|Sessionid|timespent_Sec|userid 1. Identify avg time spent per user per day 2. Create a histogram of duration on x axis, no of users on y axis which is populated by volume in each bucket Product questions: 1. Wants to add a new banner on FB Market place which suggests to sell something on facebook , should they do it, what should be done to evaluate it ? 2. Market place transactions happen through messages - these messages went down by 10% in a month, should they be worried ? How would you go about analyzing it?