1 Program to find a value from the list and replace it:

list1=[5,10,15,20,25,50,20]
index=list1.index(20)
list1[index]=200
print(list1)

[5, 10, 15, 200, 25, 50, 20]

2 Program to remove all the occurrences of a particular item from a list:

list1=[5,10,15,20,25,50,20]
while 20 in list1:
    list1.remove(20)
print(list1)

[5, 10, 15, 25, 50]

3 program to iterate two lists simultaneously while returning items in orginal and reverse orders for list1 and list 2 respt:

list1=[10,20,30,40]
list2=[100,200,300,400]
for a,b in zip(list1,list2[::-1]):
    print(a,b)

10 400
20 300
30 200
40 100