Reflection: Intervention and Extension Finding Compatible Numbers to Check Subtraction - Section 3: Student Practice


Today, I had the perfect opportunity to provide an extension for one student in particular. While solving the last problem on the subtraction practice page, he pointed to the tape diagram and said, "One whole is 24 hours." I responded, How do you know? 

Through conversation, we decided that he could check himself by dividing 86,400 seconds by 24 to get the number of minutes. Then he could divide that number by 60 again to get the number of seconds in a minute. He quickly went to work and showed me his thinking. He then showed me how he could check himself by multiplying 60 by 60 and then again by 24 to get the number of seconds. 

Then, I challenged him to find the number of seconds in half a day. It didn't take him long. He said, "I put in 86,400 divided by 2 and I got 43,200." I then said, "Can you find another way?" This time, he sat down at his desk a bit stumped. After some time, he called me over to explain his thinking: How many seconds are in half a day.MOV. The video is cut off, but here's what he did: 86,400 x 2 172,800 and then he divided this product by 4 to get 43,200. 

Although he didn't complete the whole assignment today, the sense of accomplishment and opportunity to explore outside the boundaries of a practice page mattered much more. 

  Intervention and Extension: Reflection
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Finding Compatible Numbers to Check Subtraction

Unit 5: Adding & Subtracting Large Numbers
Lesson 2 of 16

Objective: SWBAT use compatible numbers to check for reasonableness when subtracting multi-digit numbers.

Big Idea: Students will round the subtrahend and minuend to numbers that work well together to determine if an answer is reasonable.

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