In this exercise we need to calculate our first retention rate. More specifically, D7 retention rate for all mobile signups from the day of Feb 1, 2018. 
Consider a user retained if there’s any mobile event on the D7 day.
As usual, submit the integer part of the rate. For example, if the retention rate is 38.999% submit 38 as the answer.
About SQL Habit
Hi, it’s Anatoli, the author of SQL Habit. 
SQL Habit is a course (or, as some of the students say, “business simulator”). It’s based on a story of a fictional startup called Bindle. You’ll play a role of their Data Analyst 
and solve real-life challenges from Business, Marketing, and Product Management.
SQL Habit course is made of bite-sized lessons and exercises (you’re looking at one atm). They always have a real-life setting and detailed explanations. You can immediately apply everything you’ve learned at work. 
