If all the integers from $1$ up to $n$ are multiplied together, this is called the factorial
of $n$, and is written as $n!$.
For example, $4! = 4 \times 3 \times 2 \times 1 = 24$.
This means that the last-but-one digit of $4!$ is $2$.
What is the last-but-one digit of $99!$?
If you liked this problem, here is an NRICH task
which challenges you to use similar mathematical ideas.