The following sequence continues indefinitely:

27 = 3 x 3 x 3
207 = 3 x 3 x 23
2 007 = 3 x 3 x 223
20 007 = 3 x 3 x 2223.......

Which of the following integers is a multiple of 81?

a) 20 000 007
b) 2 000 000 007
c) 200 000 000 007
d) 20 000 000 000 007

If you liked this problem, here is an NRICH task which challenges you to use similar mathematical ideas.