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?
February 2010 newsletter
