想将test_number.txt里面的数字计算的平均值和合计的结果写入到test_w.txt里面，想问一下这里哪里出错了？

with open('test_numbers.txt', encoding='utf_8') as f:
    sum = 0
    n = 0
    while True:
        s_line = f.readline()
        if not s_line:
            break
        sum += int(s_line)
        n += 1
with open('test_w.txt', mode='w', encoding='utf_8') as f:
    f.write("合計 = ", str(sum)；"平均 = ", str(sum/n)