Rounding numbers – don’t teach it like this

Question: Round 478 to the nearest Hundred.

Common way to teach it:

1. Underline the Hundred place value.
2. Look at the number to the right of it (7 for the above example)
3. Is that number smaller than 5?
4. If that number is smaller than 5, keep keep the Hundreds place value as is and set the rest to 0.
5. Otherwise, increase the Hundreds place up 1 (becomes 5 in the above example), and set the rest to 0.
6. Answer: 500 for the example above.

Thee steps work, the but the student has not developed his number sense one bit. Worse, it turns math into memorizing one multi-step procedure after another. What fun is that? We do that over and over again and then wonder why our kids hate math.

Marie Montessori is know to have said, “If you want a young child to learn something, put it in their hands.” Same idea for Math. If you want a child to develop number sense, the child must be able to “see” the math.

I’ve found this visual introduction to the concept of Rounding Numbers much more useful:

Instructor: What are some “Hundreds” numbers you know?

Student: 100, 200, 300, 400, 500.

Instructor: The number 478 is between which two Hundred Numbers?

Student: that’s between 400 and 500.

Instructor: (Showing 400 and 500 on a number line) What’s the number in the middle?

Student: 450 (If student cannot answer this even with some prompts, she should not be rounding numbers)

Instructor: The number 478 is on which side (point to the left or right of 450). Which Hundred number is it closest to?

THAT, ladies and gentlemen, is the soul-satisfying explanation of the concept of Rounding.

And here’s the most amazing thing: When kids can see the math, they understand the concept in a deeper way. After some practice, they will start coming up with their own shortcuts, like “Is the digit after the Hundreds place value 5 or greater?“