How many positive 3-digit integers have a unit (ones) digit of 9? how can i solve this in the easiest way possible?

Question

mathmath333 · Answer

http://qa.pwnthesat.com/post/90669531885/how-many-positive-3-digit-numbers-have-a-units-digit-of?route=%2Fpost%2F%3Aid%2F%3Asummary

anonymous · Answer

We can use Arithmetic Sequence where...
a1=109
a2=119
a3=129...and so forth up to...
an=999, the last term
therefore common difference d=10
what is n?
an=a1+(n-1)d
999=109+10(n-1)
999-109=10n-10
900=10n
n=90