Find the sum of 35 and 15.
When we see the word sum or the
phrase find the sum, we know that means we need to add. We have to find the sum of our two
two-digit numbers. The number 35 has three 10s and
five ones, and the number 15 has one 10 and five ones. And this has been modeled for us in
our place value table. When we add two two-digit numbers,
we always have to add the ones first. Both of our two-digit numbers have
five ones. What is five plus five? Five plus five is 10.
If the ones total 10 or more, we
have to regroup. We can make a group of 10. And then we can swap our 10 ones
for one tens block. So now we have a total of no
ones. And we’ve got an extra 10 to add
with the rest of our tens blocks. We know that 35 has three 10s and
15 has one 10. Three 10s plus one more 10 gives us
a total of four 10s, plus the 10 we regrouped gives us a total of five 10s. Five 10s are 50. So 35 plus 15 equals 50.
First, we added the ones. We had 10 ones, so we
regrouped. Then we added all our tens. There were five 10s. So 35 plus 15 equals 50.