import math


def determine_fraction(pos, bin):
    """
    takes nucleotide position and binsize
    return a string of the range of the bin
    """
    frac_num = math.ceil(pos/bin)
    frac_top = (frac_num*bin)
    frac_bot = (frac_top - bin)
    return str(frac_bot) + '-' + str(frac_top)