Grade from a mark

A school grades O-Level marks like this: 80-100 = 'A', 70-79 = 'B', 60-69 = 'C', 50-59 = 'D', below 50 = 'U'. Write a function grade_for_mark(mark) that returns the grade letter for a mark. Hint: Work from the top band downwards with if / elif / else.