1. You have given a Python list. Write a program to find value 20 in the list, and if it is present, replace it with 200. Only update the first occurrence of an item.

list1 = [5, 10, 15, 20, 25, 50, 20]

list1[3]=200
print ("Updated list: ", list1)

Updated list:  [5, 10, 15, 200, 25, 50, 20]

2. Given a Python list, write a program to remove all occurrences of item 20.

list2 = [5, 20, 15, 20, 25, 50, 20]
ele=20
x=[i for i in list2 if i!=ele]
print(x)

[5, 15, 25, 50]

3. Given a two Python list. Write a program to iterate both lists simultaneously and display items from list1 in original order and items from list2 in reverse order.

list1 = [10, 20, 30, 40]
list2 = [100, 200, 300, 400]

for x, y in zip(list1, list2[::-1]):
    print(x, y)

10 400
20 300
30 200
40 100