Third graders at Laburnum Elementary have been learning how to round numbers to the nearest ten and hundred (SOL3.1b) so today I showed the students in Ms. Collins’ and Ms. Ford’s classes how to make a rounding calculator. First, for a warm-up activity, we went to Google classroom and I posted an announcement that said, “What is a number that will round to 40?” The students clicked “Add comment” and posted their ideas (any number between 35-44). Next I asked them if they thought computers could round faster than they could. We discussed that even though computers are faster than humans, they aren’t smarter. People have to program computers, and today, I explained, we were going to program the computer to round for us. I gave them a template that you are welcome to copy (a finished sample is on Sheet 2). First they could write whatever 2- or 3-digit numbers they wanted down column A. I told them to try to make it hard for the computer. Next in cell B2 we wrote the rounding formula =ROUND(A2,-1) which rounds the number in cell A1 to the tens place (the -1 means one place to the right of the decimal). Then I showed them how to drag the little blue square in the corner of B2 down through the rest of column B. It automatically rounded all the numbers in column A! Finally I asked them to figure out and test a formula in C2 that would round A2 to the nearest hundred. Many of them realized that it was =ROUND(A2,-2). If you have time, you could go on and write a formula that would round the numbers in column A to the nearest thousand.
