Estimate how many balls you see!

For actual number,
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Students with poor number sense are not able to "see" how big or small numbers are. Teach students how to group objects visually to estimate the size of the group (the number). I will often project this onto a whiteboard and draw directly on the board, showing groups of about 10. For larger numbers, I show how to look for groups that are about 10 wide and about 10 high.

Initially, students often make very explicit guesses, e.g. 23 or 87. I encourage them to make "round" guesses ending in 0 (or 5 for smaller numbers). The whole point of estimating with round numbers is to make mental math easier. If students are not able to choose/identify round numbers that are reasonably close to the actual numbers, they will find it more difficult to compute mentally.