multiples.py

def get_multiples(l, n):
    result = []
    for i in l:
        if i % n == 0:
            result.append(i)
    return result

def union(l, m):
    for i in l:
        if i not in m:
            m.append(i)
    return m

def sum_list(l):
    total = 0
    for i in l:
        total += i
    return total

l = range(1, 1000)

multiples = union(get_multiples(l, 3), get_multiples(l, 5))

print(sum_list(multiples))