55 plus 17 equals what?
We’re adding together two two-digit numbers. Two-digit numbers have a tens digit and a ones digit. 55 has five tens, which are worth 50. And 17 has one ten, which is worth 10. And 55 has five ones. 17 has seven ones. We can start by adding together the ones: five and seven. Five plus seven is 12. Now we can add the tens. Five tens and one ten makes six tens. Six tens are 60.
Now we can add together the 60 and the 12 to give us our total. 60 and 10 is 70. 12 is 10 and two more. So now we need to add the two. 70 plus two equals 72. 55 plus 17 equals 72.
We partitioned each of our two-digit numbers into tens and ones. First, we added the ones, then we added the tens, and then we added them all together. 55 plus 17 equals 72.