Teacher: Who can explain the answer to the problem?
Jose: I got 10. I started at 29 and counted up to 39: 30, 31,
32, 33, 34, 35, 36, 37, 38, 39.
Teacher: Let's write that on the chart and write Jose's
name next to that strategy. Who solved this problem a
different way?
Michael: I used the 100 Chart. I found 29 on the chart and I
saw that 39 is straight down one row. You said that
when you move straight down, you're adding 10. I
wanted to make sure, so I counted the numbers
between 29 and 39 and the answer was 10 more.
Teacher: That's great! We'll write Michael's name next to
his strategy. Who else has a different strategy for
solving this problem?
Nila: For 39, I made 3 trains of 10 cubes and 9 cubes by
itself. To take away 29, I put 2 trains of 10 and the 9
cubes in one pile and I had one train of 10 left over for
the other pile. So, if the king needed 39 cubes and he
had 29, then he needed 10 more to get to 39.
Teacher: So, you did it by taking away 29 cubes from the
39. What would your number sentence be?
Nila: If I "take away," my number sentence is 39 − 29 = 10,
but I could also say 29 + 10 = 39.
Teacher: Very nice, Nila. Both of those number sentences
describe your two piles of cubes, don't they? I'll write
Nila's name next to her strategy. Is there another way
to solve this problem?
Jason: I did it in my head. I knew that I had to have one more
for 29 to be 30 and I know that 30 + 9 = 39.
So 1 + 9 = 10.
Teacher: That's great, Jason. I'll write your name next to
your strategy. What is your number sentence?
Jason: 29 + 10 = 39 or 29 + 1 + 9 + 39.
Teacher: Good job! Is there another way to use your head
to solve this problem?
Grace: I knew the answer was 10 because when you keep the
ones the same but just make the tens number one
more, that's adding 10. If you change the tens number
two more, that's adding 20.
Teacher: You're a great thinker. We'll write Grace's name
next to her strategy. Which strategy is the easiest for
you to use?
Brenda: I think Nila's strategy is the easiest. When you do it
with cubes, it's easy to see if you have the right answer.
Teacher: That's a good point. Nila's strategy may be the
easiest to understand. How about the most efficient
strategy?
Luis: I think Jason and Grace had the most efficient
strategies because they used their heads. They didn't
have to count out all the cubes. Their way is faster.
Michael had a good strategy, too, but sometimes you
don't have a 100 Chart with you.
