用Python编写函数，传入一个自然数n，输出所有小于n的整数中能够同时被5和7整除的最大整数

def max_num(n):
for i in range(n-1, 0, -1):
if i % 5 == 0 and i % 7 == 0:
return i
return None

print(max_num(50)) # 35
print(max_num(100)) # 98
print(max_num(20)) # None