Learning to assess the reasonableness of an answer is an important mathematical skill. It’s your child’s way of seeing if she’s on the right track when problem solving. Sometimes we use rounding to estimate a solution.
In third grade, your child will round whole numbers using a vertical number line and round to the nearest ten or to the nearest hundred.
Let’s round seven-hundred sixty-two to the nearest hundred. Your child knows seven-hundred sixty-two is made up of seven hundreds, six tens, and two ones. Seven hundreds is seven-hundred. So seven-hundred-sixty-two will fall somewhere above seven-hundred on the vertical number line.
How many hundreds come after seven hundreds? Five-hundred, six-hundred, seven-hundred, eight-hundred… Eight hundreds!
Next, your child will find the midpoint or halfway mark. What falls halfway between 700 and 800? This can be tricky, so your child may skip count by fifty. Six-hundred, six-hundred fifty, seven-hundred, seven-hundred fifty, eight-hundred… Seven-hundred fifty is the midpoint!
Ask your child: Where will you place seven-hundred sixty-two on this number line? Ummm… Here! Just a little above the midpoint.
Using a vertical number line is a very helpful model. Your child can clearly see that seven-hundred sixty-two is closer to eight-hundred than it is to seven-hundred, so it rounds up to eight-hundred.
Seven-hundred sixty-two rounded to the nearest hundred is eight-hundred. Or, seven-hundred sixty-two is approximately equal to eight-hundred.
Talk with your child about this special case: When a number falls exactly on the midpoint, you round up. Like this - twenty-five is the midpoint, and twenty-five rounded to the nearest ten is thirty because you round up. Twenty-five is approximately thirty.
Using a vertical number line gives your child a visual representation for rounding. With practice, she will always see when to round up and when to round down.
And that’s good to know.