This is an activity I’ve done with my 5th, 6th and 7th graders to help them understand the importance of “thinking” before rounding off a number. You know, some of us (not you, of course) teach rounding as a “rule” and our students blindly go off rounding numbers without thinking about the implication of doing so. These two activities are designed to impress upon your students that when they round off a number, they should do it with a certain amount of forethought, because if they blindly follow the “rule,” they’ll end up with the wrong answer!
Well, mathematically the answer is correct, but the result will be all wrong. In this case, the context is fencing in an area of ground. Now, if you need to fence off an area and you order too little fencing, then that’s not going to work, right? That’s why I designed this actvity so that if you blindly follow the rules of rounding, your students will end up ordering too little fencing. HA!
I like to let students work in pairs on these kinds of activities and then discuss their results with others. It’s important that they have a chance to see the implications of blindly following a “rule” and that in mathematics you have to go with something more intuitive than rules and more thoughtful than “common sense.” YOU HAVE TO THINK BEFORE ACTING, EVEN THOUGH YOU HAVE THE RIGHT ANSWER!