Write SQL query to solve given problem: What is the ratio of male and female users in 27-28 age group?. Keep the solution inside sql tag ```sql [SQL-Query] ```
SELECT SUM(IIF(gender = 'M' AND `group` = 'M27-28', 1, 0)) / SUM(IIF(gender = 'F' AND `group` = 'F27-28', 1, 0)) AS r FROM gender_age