repeat = True
#Prompt and read the two numbers
while repeat:
    firstNumber = float(input("Enter the first number: "))
    secondNumber = float(input("Enter the second number: "))
    #Calculate the sum
    sum = firstNumber + secondNumber
    print("The sum is:", sum)
    #The loop should ask the user if he or she wishes to 
    #perform the operation again. If so, the loop should
    #repeat, otherwise it should terminate.
    userChoice = input("Do you want to perform the operation again? (y/n): ")
    if userChoice.lower() != 'y':
        repeat = False

Executed Output:

Enter the first number: 20
Enter the second number: 300
The sum is: 320.0
Do you want to perform the operation again? (y/n): y
Enter the first number: 25
Enter the second number: -5
The sum is: 20.0
Do you want to perform the operation again? (y/n): n