Challenge Level

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 $9$.

You can find more short problems, arranged by curriculum topic, in our short problems collection.