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

index = list.index(20)

list[index] = 200

print(list)

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

list = [10,20,45,20,50,15]

def removeValue(NewList,val):
  return [value for value in NewList if value != val]
result= removeValue(list,20)
print(result)

[10, 45, 50, 15]

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