The ideas in this addition lesson are taken from Add & Subtract 2-B ebook.

Rounding to nearest ten

Number line from 210 to 310

Is 223 nearer to 220 or 230?
Is 247 nearer to 240 or 250?

Is 256 nearer to 250 or 260?
Is 283 nearer to 280 or 290?

Is 298 nearer to 290 or 300?
Is 305 nearer to 300 or 310?

The symbol ≈ is read "is approximately to". In the example above,

223 ≈ 230
247 ≈ 250

256 ≈ 260
283 ≈ 280

298 ≈ 300
305 ≈ 310

This process is called rounding to the nearest ten. Actually, 305 is equally close to 300 and 310, but it has been agreed that numbers ending in 5 are 'rounded up' or rounded to the next ten.

When you are rounding to the nearest ten, the rules are:

If a number ends in 1, 2, 3, or 4, then round down.
If a number ends in 5, 6, 7, 8, or 9, then round up.

Examples (rounding down):

671 ≈ 670, because 671 ends in 1	423 ≈ 420, because 423 ends in 3
544 ≈ 540, because 544 ends in 4	202 ≈ 200, because 202 ends in 2

When you round up, then tens digit increases by one. Examples:

766 ≈ 770, because 766 ends in 6. Tens digit (6) becomes 7.	435 ≈ 440, because 435 ends in 5. Tens digit (3) becomes 4.
705 ≈ 710, because 705 ends in 5. Tens digit (0) becomes 1.	296 ≈ 300 because 296 ends in 6. Tens digit (9) would become 10, which means we have 10 tens, a hundred. So the number is rounded to the next whole hundred.

Try to 'visualize' or 'see' the number line in your mind when rounding to nearest ten.

Practice

1. Place the numbers on the number line and round them to the nearest whole ten.

554 ≈ ____

628 ≈ ____

551 ≈____

641 ≈ ____

585 ≈____

625 ≈____

595 ≈ ____

592 ≈ ____

636 ≈____

567 ≈ ____

604 ≈ ____

605 ≈____

612 ≈ ____

616 ≈ ____

597 ≈____

2. Round these numbers to the nearest ten.

78 ≈ ____

64 ≈ ____

86 ≈ ____

139 ≈ ____

98 ≈ ____

105 ≈____

233 ≈ ____

887 ≈ ____

654 ≈ ____

283 ≈ ____

403 ≈ ____

566 ≈ ____

347 ≈ ____

816 ≈ ____

46 ≈ ____

705 ≈ ____

288 ≈ ____

497 ≈ ____

908 ≈ ____

202 ≈ ____

We can use rounding to estimate sums and differences.

Estimate the sum 78 + 54.
Since 78 ≈ 80 and 54 ≈ 50, 78 + 54 ≈ 80 + 50 = 130.

Estimate the sum 423 + 69.
Since 423 ≈ 420 and 69 ≈ 70, 423 + 69 ≈ 420 + 70 = 490.

Estimate the difference 377 − 45.
Since 377 ≈ 380 and 45 ≈ 50, 377 − 45 ≈ 380 − 50 = 330.

3. Estimate these sums and differences.

a. 148 + 43 ≈ 150 + 40 = 190.	b. 678 + 45 ≈ ___ + ___ = ___	c. 89 + 56 ≈ ___ + ___ =___	d. 344 + 34 ≈ ___ + ___ = ___
e. 237 + 52 ≈	f. 66 + 42 ≈	g. 23 + 98 ≈	h. 458 + 31 ≈
i. 178 − 43 ≈	j. 278 − 56 ≈	k. 873 − 98 ≈	l. 771 − 37 ≈

4. Solve the word problems using estimation. Use rounding to estimate the needed sums or differences.

a) In September Mom used the Internet for 53 hours, in October for 28 hours, in November for 19 hours, and in December for 35 hours. How many hours approximately did she use it total during those four months?

b) Susan bought 4 pens. The first pen cost 29 cents, the second one 32 cents, and the third one 44 cents. She bought two of the first kind. Estimate how much money did the pens cost total.

c) Jack, Ben, and Rick have been helping Grandpa to do yard work during the summer. Jack has earned 39 dollars, Ben has earned 27 dollars, and Rick has earned 122 dollars.

How much more approximately has Rick earned than Ben?

How much more approximately has Rick earned than Jack?

How much have they earned approximately all together?

d) You have $180. Estimate: do you have enough to buy a sofa for $118 and a table for $45?

How about if you buy a lamp for $29, four chairs $19 each, and a table for $65?

How about if you buy a table for $57, a bed for $110 and a lamp for $15?

FREE math homework help on www.PhatMath.com

40 Math forums for grades K-12 & College
Post/view math problems & solutions
Blogs for students, parents, teachers

Learn fractions visually
Self-teaching worktext
Download and print

See free samples!

Times Tales

Learn the upper
times tables with
mnemonic stories.

Learn more!

HOMESCHOOL MATH NEWSLETTER

Homeschool Math Newsletter comes out once monthly, and includes math teaching articles and tips, Math Mammoth news, and all kinds of little "tidbits" such as math news, interesting links, or humor. The content is equally good for all of us who teach math (not just homeschooling parents).

When you subscribe, you will alsoreceive a GIFT of over 280 FREE math worksheets and sample pages from Math Mammoth books for grades 1-8.

Note: This is a double opt-in list. You will FIRST get an email that asks you to confirm your email address. PLEASE check also your SPAM/JUNK folder for this confirmation email.

The newsletter service is by Aweber.com. Your email address will not be shared, sold, or distributed to anyone else.

Check out these short video clips from my favorite band - Rockabilly.US. They play oldies, Elvis, country, and top hits of the 50s and 60s. It's a show with large band, costumes, choreography, little skits, "Kid Kritters", 6 dancing/singing girls, junior dancer troupe, etc.
"Be My Baby" originally by the Ronettes	Impossible Dream (Elvis song)
Most importantly, the band wants to use the uplifting and energetic music to bring some fun, happiness and encouragement into people's lives. They're doing it selflessly to help others, and don't get paid for any of it themselves. And I admire that! => See more video clips at YouTube