Python所有运算符优先级的操作：
a = 20
b = 10
c = 15
d = 5
e = 0
e = (a + b) c / d #( 30 15 ) / 5
print ("(a + b) * c / d 运算结果为：", e)
e = ((a + b) c) / d # (30 15 ) / 5
print ("((a + b) * c) / d 运算结果为：", e)
e = (a + b) (c / d) # (30) (15/5)
print ("(a + b) * (c / d) 运算结果为：", e)
e = a + (b c) / d # 20 + (150/5)
print ("a + (b c) / d 运算结果为：", e)
