1.
l = [5,10,15,20,25,50,20]
for i in range(len(l)):
if(l[i]==20):
l[i] = 200
break
print(l)

2.
l = [5,20,15,20,25,50,20]
ans = []
for i in l:
if(i != 20 ):
ans.append(i)
print(ans)

3.
list1 = [10,20,30,40]
list2 = [100,200,300,400]
for i in range(4):
print(list1[i],list2[4-i-1])