def format_duration(duration):
    """Given a duration in minutes, return a string on the format h:mm.
    >>> format_duration(75)
    '1:15'
    >>> format_duration(4)
    '0:04'
    >>> format_duration(601)
    '10:01'
    """
    h = duration // 60
    m = duration % 60
    return f"{h}:{m:02}"