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The ideas in this addition lesson are taken from Math Mammoth Add & Subtract 3 book.


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 ≈ 220
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, one 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.

a. b. c. d. e.
554 ≈ ____

628 ≈ ____

551 ≈____

641 ≈ ____

585 ≈____

625 ≈____

595 ≈ ____

592 ≈ ____

636 ≈____

567 ≈ ____

604 ≈ ____

605 ≈____

612 ≈ ____

616 ≈ ____

597 ≈____




2.  Round these numbers to the nearest ten.

a. b. c. d. e.
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?





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