Rounding to Check Accuracy

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Students will be able to round to the nearest ten or hundred in order to evaluate the reasonableness of their final answer.

Big Idea

In order to approximate a total, or to help check the reasonableness of an answer, students need to be able to round to the nearest 10 or 100. Check out this lesson for a fun game that gives them practice rounding, adding, and subtracting.


15 minutes

As a mini-lesson, I teach the students a new game.  I ask the children to form a fishbowl in our community center and ask a volunteer to be my partner.  

I give my partner and myself a Rounding Game Board and a set of directions.  We read the directions aloud and paraphrase what we will be doing.  Then, we follow the directions.  While we play, I think aloud about my strategy for making my three digit numbers, based on the operation we are completing.  

My partner and I also discuss how we use rounding to check for accuracy.  It is here that I ask the fishbowl to discuss with a partner next to them if they think we should round to the nearest 10 or 100 and why?  

I guide the conversation around the fact that both strategies would work, but that my partner and I will model each with the same problem.  Afterwards, the class should see that rounding to the nearest 10 gives us a more accurate estimate.

Active Engagement

25 minutes

I quickly pair the students using our name sticks and send them off with the directions and game boards.  As each group assembles their playing cards, I meet with a few students that I know are still struggling with why we round numbers the way we do.  I will do a quick demonstration, again, on the number line about the distance between the decades and century landmarks for these students and then send them to their partners. 

As the partnerships play, I join teams in order to watch for strategies in creating 3 digit numbers and in rounding for accuracy.  I think it is also important to watch and listen to the strategy development for adding and subtracting the actual numbers.

In this clip, you will see that the student is able to explain her rounding method and use it to check her actual work.  Prior to the filming, however, she was confused about how to round.  I intervene using a number line partitioned into intervals of 10's, and ask her to "plot" numbers I call out.  I then review when to round, based on distance from a landmark number.



10 minutes

To close the lesson, I placed a three digit subtraction problem on the board and ask a student to show me the estimate equation using their white boards.  This acts as an exit ticket. When I tap them on the shoulder, they are made aware that their equation is correct and can go on to practice their multiplication/division flashcards.

If their equations are off, I form a small group and do another intervention of how to choose the landmark 10's or 100's, and then discuss the distance from each as a way of knowing which direction to round.  I also give this group the rule: 4 and lower in the ones place, round down.  5 and higher in the ones place, round up.