Filtering grades based on city avg, in SQL

Given the following table, called 'students', write a SQL query to count the # of students with grades above the Detroit average.

	age	city	grade	name
0	20	Detroit	88	Willard Morris
1	19	Detroit	95	Al Jennings
2	22	New York	92	Omar Mullins
3	21	San Francisco	70	Spencer McDaniel