RoundingDecimals
Suppose
therewere5362spectators
at an Olympicswimmingevent.
Roundthisnumberto the nearest
ten andnearest
hundred.
Whatstrategies
did you use?
Youcanusethe samestrategies
to rounddecimals.
T h i st a b l esh o w sth e w i n n i n gti me sof sevengold m edal
w i n n e r sa t t h e 2 0 0 0S yd n e yOl ympics.
Event
Time(seconds)
Athlete
Women's50-m FreestyleSwimming
t. D e B r u i j n
24.32
Men's50-m FreestyleSwimming
G . H a l lJ r .a n d A . E r v i n
21.98
Women's100-mDash
M.Jones
10.75
Men's100-mDash
M.Greene
9.87
Women's100-mFreestyleSwimming
t. De Br uijn
53.83
Men's100-mFreestyleSwimming
P.van den Hoogenband
48.30
S u p p o s et i me sw e re re co rd e di n w hole seconds.
What would each person'stime be?
U s ea n y m ate ri a l yo
s u n e e dto h e l pyou.
ShOUV ,.:ii.i::':l-:':
Shaf
e
S h a r ey o u r ro u n d e dti me sw i th a cl assm ate.
D i s c u s tsh e stra te g i eyo
s u u se dto r ound.
W h y m i g h t yo u w a n t to ro u n dth e times
t o t h e n e a re stse co n d ?
| 24
LrssoNFocus
Mental Math
Onebookcosts$10.
w o u l de a c hn u m b e r
H o wm u c h
of bookscost?
Rounddecimolsto the neorestwhole number
7
35
128
4327
)
ln the women's100-mdash,the gold medaltime was 10.75seconds.
Thesilvermedaltimewas11.12seconds.
Thesetimesaremeasured
to the nearesthundredthof a second.
Towritean estimateof thesetimes,you canroundto the nearestsecond.
Hereare2 waysto round10.75and 11.12to the nearestsecond.
. Usea numberline.
10.75
10
11.12
10.50
10.75is between
10 and 11.
butcloserto 11.
So,10.75secondsroundsup
to 11 seconds.
1 1. 5 0
12
11.12is between11 and 12.
b u t c l o s e rt o 1 1 .
S o , 1 1 . 1 2s e c o n d sr o u n d s
downto 11 seconds.
. Usewhat you know about decimalhundredths.
Lookat the hundredths
in 10.75.
Youknowthat 75 hundredths
is closer
to 1 thanto 0.
So, 10.75roundsup to 11.
Lookat the hundredths
in 11.12.
Youknowthat 12 hundredths
is closer
to 0 thanto 1.
s o w nto 1 1.
So ,1 1 .1 2ro u n d d
D Oneswimmerhada timeof 56.50seconds
in the men's100-mbackstroke.
To round56.50seconds
to the nearestsecond:
is halfwaybetween0 and 1,youroundup.
Since50 hundredths
So,56.50
seconds
roundsto 57 seconds.
r25
1. Roundto the nearest
wholenumber.
al 4.78
sl 7.49
b) 6.31
h l 1 1 .7 1
c) 5.09
i ) 40.12
Roundto the nearest
dollar.
a) Sq.t8
b) 51.22
e) 50.2t
fl 54.49
d) 1.98
i) 4.47
el 3.2
kl 1.25
c) 5z.so
d) 53.99
d 50.e7
h)s21.ss
3. Whichdecimals
couldbe roundedto eachcirclednumber?
Explain.
a) @ 6.42, 7.1, 6.08
b) o 8.50,9.43,9.01
c ) @ 1 . 9 , 2 . 8 1, . 5 0
d) @ 0.09,3.89,4.44
a. Thetooniehasa massof 7.3g anda thickness
of 1.Bmm.
Roundeachmeasurement
to the nearest
wholenumber.
5. The1908pennyhasa massof 5.67g anda widthof 25.4mm.
Roundeachmeasurement
to the nearest
wholenumber.
6. Thebiggestearthwormeverfoundwas6.7m longand2.03cm wide.
Roundeachmeasurement
to the nearest
wholenumber.
7. A runner's
timein the 100-mdashisabout11 seconds
whenroundedto the nearestsecond.
Suppose
the timewasrecorded
to the nearesthundredth
of a second.
Whatarethe fastestandslowestpossible
times?
--?t:4W-*
7*
8 . U s et h e d i g i ts4 ,5 , a n d6 .
a ) W r i tea s ma n yd e ci ma l sa syou canthat
roundto 5 whenroundedto the nearestwholenumber.
b) Orderthe decimals
from leastto greatest.
c) Suppose
the digit5 wasreplaced
with 3.
Howwouldthisaffectyourresults?
Explain.
place.
Choosea numberwith a digit in the hundredths
Howwouldyou roundyournumberto the nearest
wholenumber?
| 26
Ass:ssmrurFocus i Quesrion
8
f)
r)
12.50
3.62
ShowWhatYouKnow
LESS0il
i
UseBaseTenBlockswhenthey help.
i
:
i
1. Writeeachfractionor mixednumberasa decimal.
.)+
b) 3#
d 201fu
i
:
2. Usethe digits1,3,4,8,
anda decimalpoint.
Writethe numberthat is closestto 40.
D
3. Writean equivalent
decimalfor eachdecimal.
;
al 0.8
bl 7.20
c) 1.1
dt#
dl 0.60
i
i
i
4. al Whenwouldyou write7.50insteadof 7.57
U) Whenwouldyou write7.5insteadof 7.50?
Usethe digits3,4,5,anda decimalpoint.
Writeallthe possible
decimals.
Orderthesedecimals
from leastto greatest.
:
:
I
i
Usethe digits0,5,8in theseboxes:tr.
n n
a) Writethe greatestnumber.
b) Writethe leastnumber.
i
F
JI
-l
::
:
i
i
:
D
:
Copyandcomplete.
Use), (, or =.
a) 3.04fl 3.19 b) 0.40tr 0.4
cl 1.7a 1.2s
8. Writethe decimals
in orderfrom greatestto least.
a l 5 . 6 2 , 5 . 95
, .30
bl 0.95,0.6, 1.3
Copyeachstatement.
Writea decimalto makethe statement
true.
a)0.85>tr
bI 7.62<n
c} 0.04>n
10-Roundeachdecimalto the nearestwholenumber.
al 2.34
b) 0.6
c) 0.84
d) 5.5
11. An athlete's
longjump,roundedto the nearestmetre,was8 m.
Whatarethe longestand shortestlengthsthe jump
couldhavebeen,in decimals
to the hundredths?
148