If I understand the problem correctly, I would ask a user what operation he wants to go with:
math_option =raw_input('Choose the math operation: ')
and later on check what option was chosen:
if math_option == "add":
print add(x, y)
if math_option == "multiply":
add_num = add(x,y)
mul_num = multiply(math_option,z)
print mul_num
and so on
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solved Basic Python 2.7 Math Script (4 variables and 3 consecutive math operations) [closed]