Joe Schmoe says that 15 − 2 = 16 − 1 is a true
number sentence. What would you tell him? Talk
to a partner first, then share your thoughts with
the class. (Possible responses: Joe is incorrect.
For a number sentence to be true, both sides have
to be equal. If I start at 15 and count back 2, I get
13. If I start at 16 and count back 1, I get 15.
They are not the same.)
Display the number sentence 15 = 9 + 1 + 5 and say:
Joe's sister Jane is finding missing numbers. She
says that 15 = 9 + 1 + 5 is a true number sentence.
Do you agree? How do you know? (yes;
Possible response: You can put 9 and 1 together
to make 10 and then count on 5 more: 11, 12, 13,
14, 15.)
How many more true sentences can you write for
15 = _______? Use your tools to help. (Some possible
responses: 15 = 10 + 5; 15 = 9 + 6;
15 = 16 − 1; 15 = 7 + 7 + 1)
