Powerful 9
What is the last digit of this calculation?
Answer: $9$
Look at the last digits of powers of $9$.
odd even
$9$ $81$
$729$ $ 279\times 9 = ....1$
$ ...1\times9=...9$ $...1$
$...9$ $...1$
$9^9$ is a power of $9$ so $9^9$ is odd
This means that $9^{9^9}$ is an odd power of $9$, so it ends in $9$.