In this tutorial, we'll start adding and subtracting numbers that have two (yes, two!) digits. We won't be doing any carrying or borrowing (you'll learn what those are shortly) so you can see that adding or subtracting two digit numbers is really just an extension of what you already know.
You're somewhat familar with adding, say, 17+12 or 21+32, but what happens for 13+19? Essentially, what happens when I max out the "ones place". In this tutorial, we'll introduce you to the powerful tool of carrying and why it works.
You can subtract 21 from 45, but are a bit perplexed trying to subtract 26 from 45 (how do you subtract the 6 in 26 from the 5 in 45). This tutorial is your answer. You'll see that we can essentially "regroup" the value in a number from one place to another to solve your problem. This is also often called borrowing (although it is like "borrowing" sugar from your neighbor in that you never give it back).