What If You Had Eleven Fingers?

What If You Had Eleven Fingers?

Rounding numbers

We use ten as a basis for a lot of math activities. Because we have ten fingers and toes, it is an easy place to start. If we had eleven fingers, our math system would probably be based on eleven.

But, what if you don’t need to know the exact number of something, just close? You might want to round to the nearest number that can be divided by ten. Any number that ends with five or more (5, 6, 7, 8, 9) is rounded up to the next number that can be divided by ten. Any number between zero and four (0, 1, 2, 3, 4) is rounded down to the next number that can be divided by ten.

Materials

• 100 buttons or other small objects
• Optional: 11 hula hoops or shoe boxes or large bowls
• Optional: index cards labeled 1-99, not including the 10s
• Optional: labels for each of the hula hoops, shoe boxes or large bowls 0, 10, 20, 30, to 100

Instructions

• Pull out several small batches of buttons
• Count each batch of buttons, one at a time
• For each batch, count the number of buttons it would take to go each direction to the next ten number and see which one is closer
• For example, if you have 7 buttons in a batch, it only takes 2 buttons to get to 10 (8, 9) but it takes 7 buttons to go down to 0 (6, 5, 4, 3, 2, 1, 0). 7 is closer to 10 than to 0. We only count 0 once, as part of the lower half of the 10 numbers, with 5 counted in the upper half.
• Call out the closest ten number for each batch of buttons
• If there is a question, count the number of buttons it would take to get to the nearest ten in both directions, to see which is closer. The fewer number of buttons it takes to get to the next ten number is the closest ten you round to.
• Optional: Scatter the index cards inside all the hoops, or boxes, or bowls
• Optional: let your grandchildren loose to pick up each index card and place it inside the hula hoop it belongs in. For example, the index card labeled 97 goes inside the hula hoop marked 100, because 97 is closest to 100, or rounded up to 100. The index card labeled 34 goes inside the hula hoop marked 30, because 34 is closest to 30, or rounded down to 30.

What Should Happen?

With only a few batches ,the rounding pattern will start to become clear. Numbers 5 and above round up to the next ten. Numbers 4 and below round down to the next ten.

Let’s look at some examples.

• If there are 3 buttons in a batch, the nearest ten is 0. If you counted down from 3 to 0, you would have 2, 1, 0. If you counted up from 3 to 10, you would have 4, 5, 6, 7, 8, 9. The three buttons represented by 2, 1 and 0 are fewer than the 6 buttons represented by 4-9. Three is less than 6, so three is closer to 0 than 10. When rounding to the closest ten, 3 is rounded down to 0.

• If there are 8 buttons in a batch, the nearest ten is 10, because 8 is closer to ten than to 0.

• If there are 11 buttons in a batch, the nearest ten is 10. That is, 11 is closer to 10 than to 20.

• If there are 17 buttons in a batch, the nearest ten is 20. That is, 17 is closer to the next ten, the number 20, than it is to the lower ten, 10.

• For 25, the nearest ten is 30. You go up for five and above.

• For 34, the nearest ten is 30. You go down for numbers four and below.

Why Is This Important?

Rounding makes it faster and easier to work with several large numbers at a time.

This can come in handy when you are shopping. For instance, you might need a calculator to figure out how much two books cost that are $14.99 and $5.34. You can round to the nearest ten and quickly add $15.00 and $5.30 to get $18.30, bringing you within 3 cents of the money you will need to buy the books.

Thanks to ixl.com for this suggestion.

Thanks to lessonsplanspage.com for the hula hoop activity.

 

Carol Covin, Granny-Guru

Author, “Who Gets to Name Grandma? The Wisdom of Mothers and Grandmothers”

http://newgrandmas.com

 

 

Filed in: education Tags: featured • kids math • rounding