Display the Professor Peabody's Problem Master.
Have students use the Math Practices page in the
Student Activity Book to analyze the answer
Professor Peabody gave for the following problem.
- In the morning Maria had 90 marbles in 9 bags of
ten. She sold 70 marbles to Tony. Later she bought
20 more marbles. How many marbles did she have
at the end of the day?
- Professor Peabody said, "My number sentence is
90 − 70 = 20. I used my head. First I added
70 + 20 = 90. Then I subtracted 90 − 90 and the
answer is 0."
- Do you think Professor Peabody is right?
Have students work in pairs to discuss the reasonableness
of Professor Peabody's answer.
- Did Professor Peabody know the question to
answer and what information is important [MPE1]?
(Yes. He knew which numbers were important
and what he had to solve.)
- Did he find a strategy [MPE2]? (Possible response:
Yes, he used mental math to solve the number
sentence 90 − 70 + 20.)
- Was it an efficient strategy? (Possible response: No, because he got the wrong answer.)
- Was his answer reasonable [MPE3]? (Possible
response: No, because how could he have 0 if
Maria bought 20 more marbles?)
- What do you think he did wrong? (Possible
response: He added 70 + 20 and took that away
from 90. Instead of adding the 20 marbles Maria
bought later, he subtracted that number, also.)
- How would you solve Professor Peabody's problem?
(Solution strategies will vary.)
Professor Peabody's response should help students
to see the importance of checking their solutions for
reasonableness.
