Write one function mean_grade(stud_id, results) that accepts as input a student id and a table of study results of various students and units and that returns as output the mean grade of the student across all his/her units. The input table is expected to contain three columns: unit, student id, grade. You can assume that there is at least one result for the input student id.

For example:

>>> results = [['Database', 101028, 65],

['Database', 101022, 80],

['Operating Systems', 493968, 68],

['Operating Systems', 201022, 59],

['Java Fundamentals', 493968, 45],

['Java Fundamentals', 101022, 85],

['Mathematics', 101022, 71],

['Mathematics', 493968, 67],

['Information Systems', 493968, 75]]

>>> mean_grade(493968, results)

63.75

>>> mean_grade(101022, results)

78.66666666666667

Your solution may not use any imports.