Ответ:

1)

for i in range(1, 26, 3):

   print(i)

2)

sum = 0

x = 6

while x <= 20:

   sum += x

   x += 2

print(sum)

3)

sum = 0

x = 5

while x <= 20:

   sum += x

   x += 2

print(sum)