求助三个编程题的答案

# 1

weight = 50
for i in range(1,11):
    weight = 1.02 * weight
    print('%d years later:%.2f'%(i, weight))

输出：

1 years later:51.00
2 years later:52.02
3 years later:53.06
4 years later:54.12
5 years later:55.20
6 years later:56.31
7 years later:57.43
8 years later:58.58
9 years later:59.75
10 years later:60.95

# 2

n = int(input())
if n<=3000:
    print(n)
elif n<=5000:
    print(n*0.99)
else:
    print(n*0.97)

# 3

def niu(n):
    if n<4:
        return 1
    else:
        return niu(n-1)+niu(n-3)
n = int(input('请输入n(n>=1)：'))
print('%d年后小牛的个数为%d'%(n,niu(n)))

请输入n(n>=1)： 20

20年后小牛的个数为872