x=int(input())
if x<0:
print('0')
elif x>=0 and x<5:
print(x)
elif x>=5 and x<10:
print(3*x-5):
elif x>=10 and x<20:
print(0.5*x-2)
elif x>=20:
print(0)

3*x-5 invalid syntax 咋搞？

x=int(input())
if x<0:
    print('0')
elif x>=0 and x<5:
    print(x)
elif x>=5 and x<10:
    print(3*x-5)
elif x>=10 and x<20:
    print(0.5*x-2)
elif x>=20:
    print(0)

print(3*x-5):后面没有：