Sometimes with a math problem you just have to experiment.

What is the units digit of $9^1?$
What is the units digit of $9^2?$
What is the units digit of $9^3?$
What is the units digit of $9^4?$
What is the units digit of $9^5?$

What is the units digit of $9^{23}?$
Why?

Penny

Math Central is supported by the University of Regina and The Pacific Institute for the Mathematical Sciences.