python 语法错误 30/09/2013

我的代码在 = 部分显示“<= 80”的行返回错误。这是为什么？我该如何解决？

#Procedure to find number of books required 
def number_books(): 
    number_books = int(raw_input("Enter number of books you want to order: ")) 
    price = float(15.99) 
    running_total = number_books * price 
    return number_books,price 
 
#Procedure to work out discount 
def discount(number_books): 
    if number_books >= 51 and <= 80: 
        discount = running_total / 100 * 10 
    elif number_books >= 11 and <=50: 
        discount = running_total / 100 * 7.5 
    elif number_books >= 6 and <=10: 
        discount = running_total / 100 * 5 
    elif number_books >=1 and <=5: 
        discount = running_total / 100 * 1 
    else print "Max number of books available to order is 80. Please re enter number: "         
        return discount 
 
#Calculating final price 
def calculation(running_total,discount): 
    final_price = running_total - discount 
 
#Display results 
def display(final_price) 
print "Your order of", number_books, "copies of Computing science for beginners will cost £", final_price  
 
#Main program 
number_books() 
discount(number_books) 
calculation(running_total,discount) 
display(final_price) 

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