Q&A - PSLE Math

[Dharma]

tianzhu:

Perhaps girls of 2008/9 RGPS would like to comment on the extra cube shown in the picture.


My gut feeling is it's not there.

You're right. It was hand-drawn (its the one at the top row - on the righthand side)

Cube size - 2cm x 2cm x 2cm
Height of container = 6cm

[Dharma]

tianzhu:

Other than Guess and Check, what are the other alternatives to solve this problem?


Annie wrote her mother's age followed by her own age to form a 4-digit number.She used the difference between their ages to subtract from the 4-digit number. Then Annie obtained the number 4489.What was Annie's age ? (RGPS P6 Prelim 2008 Q47)

The 4 digit number is ABCD ... AB is mother's age while CD is Annie's age.

ABCD - (AB - CD) = 4489

ABCD = (AB00 + CD) -( AB - CD) = 4489

AB - CD > 10 .... AB = 45

4500 - CD - 45 + CD = 4489
2CD = 34
CD = 17

Annie's age = 17

[Tang]

Dharma:

Drdj:

Hi there,


Please help me with the following :

NanHua Prelim 2008

Qn 24
15 cats can catch 15 rats in 15 minutes. How long does it take 60 cats to cath 60 rats? [Ans = 15 mins - how do you know which qty to stay constant/change]

Thanks in advance

15 cats - 15 rats - 15 mins
1 cat - 15 rats - 225 mins
1 cat - 60 rats - 900 mins
60 cats - 60 rats - 900/60 = 15 mins
(Only change 1 independent variable at a time)

15 cats 15 rats 15 min
1 cat 1 rat also 15 min
Hence 60 cats 60 rats also 15 min

[Tang]

Tang:

Dharma:

[quote=\"Drdj\"]Hi there,


Please help me with the following :

NanHua Prelim 2008

Qn 24
15 cats can catch 15 rats in 15 minutes. How long does it take 60 cats to cath 60 rats? [Ans = 15 mins - how do you know which qty to stay constant/change]

Thanks in advance

15 cats - 15 rats - 15 mins
1 cat - 15 rats - 225 mins
1 cat - 60 rats - 900 mins
60 cats - 60 rats - 900/60 = 15 mins
(Only change 1 independent variable at a time)

15 cats 15 rats 15 min
1 cat 1 rat also 15 min
Hence 60 cats 60 rats also 15 min[/quote]15 cats 15 min

60 cats, the number of min will be less (Multiply by 15/60 - Original number of cats / New number of cats).
60 rats, the number of min will be more, since more rats to catch (Multiply by 60/15 - New number of rats / Original number of rats)

So 60 cats to catch 60 rats will require 15 x 15/60 x 60/15 = 15 min.

[Dharma]

Drdj:

Hi there,


Please help me with the following :

Misc
David paints once every 2 days. Larry paints once every 3 days.
If the 2 men met on 28th May, what is the earliest date David has to start work? [Answer = 24th May]

Thanks in advance

The underlying assumption is both David and Larry met for the 1st time on 28 May.

If the assumption is not true, they would have met 6 days before i.e. on 22 May (LCM of 2 and 3)

Since the assumption holds, then the earliest David starts work is on 24 May (2 days after 22 May)

[Andaiz]

Drdj:

Hi there,


Please help me with the following :

Misc
David paints once every 2 days. Larry paints once every 3 days.
If the 2 men met on 28th May, what is the earliest date David has to start work? [Answer = 24th May]

Thanks in advance

I would have assumed too that they've met for the first time on 28th May.
Larry would have worked on 28-3 i.e. 25th; and 25-3 i.e., 22nd.

With David painting every 2 days, he'd have worked 28th, 26th, or 24th or 22nd.

If they have never met previously, David's earliest date he could have started work is 24th.

We used to do the LCM way but now its modelling eh? I'd try with the number line


David 22_________ 24 _______26___________28
Larry 22_______________25_______________ 28

[enoawng]

tianzhu:

Thank you for your help.


RGPS SA2 2008 Q43.


http://www.postimage.org/

3 plane of waters
1st plane has 1 cube
2nd plane has 3 cubes
3rd plane has 6 cubes and 352cm3

2xtimes third plane = 768cm3 + 4 cubes
12cubes + 2x352cm3 is the same as 768cm3 + 4 cubes

8 cubes = 64cm3
1 cube = 2x2x2cm = 8 cm3

Hence height of cube = 2cm ; height of tray = 6cm

[Tang]

[Moderator's note: Topics merged.]


Can't recall if I have posted Uncle Observer's Blog on P6 Maths Questions here previously.

http://prischoolmaths.blogspot.com/


Anyway the answers have been released and you may want to check.


http://prischoolmaths.blogspot.com/2009/07/maths-volume_18.html

http://prischoolmaths.blogspot.com/2009/07/maths-percentage_2485.html

http://prischoolmaths.blogspot.com/2009/07/maths-percentage_4168.html

http://prischoolmaths.blogspot.com/2009/07/maths-percentage_17.html

http://prischoolmaths.blogspot.com/2009/07/maths-whole-number_16.html

http://prischoolmaths.blogspot.com/2009/07/maths-whole-number_18.html

http://prischoolmaths.blogspot.com/2009/07/maths-fraction_19.html

http://prischoolmaths.blogspot.com/2009/07/maths-fraction.html

http://prischoolmaths.blogspot.com/2009/07/maths-rates.html

http://prischoolmaths.blogspot.com/2009/07/maths-speed_22.html

http://prischoolmaths.blogspot.com/2009/07/maths-speed_18.html

http://prischoolmaths.blogspot.com/2009/07/maths-average.html

http://prischoolmaths.blogspot.com/2009/07/maths-circles.html

http://prischoolmaths.blogspot.com/2008/11/maths-challenging_01.html

[N3SKiasu]

Hi I have one Maths question but the answer kit used Algebra to solve, hence wld like to kw if anyone cld help with using Models.


Mr Tan had some apples and oranges in his shop. 30% of the fruits were oranges. He sold 70 apples and bought another 170 oranges. Then 65% of the fruits were oranges. How many apples did he have at first?

Thks in advance!

[Tang]

N3SKiasu:

Hi I have one Maths question but the answer kit used Algebra to solve, hence wld like to kw if anyone cld help with using Models.


Mr Tan had some apples and oranges in his shop. 30% of the fruits were oranges. He sold 70 apples and bought another 170 oranges. Then 65% of the fruits were oranges. How many apples did he have at first?

Thks in advance!

After
-----
Oranges New -- 3 units + 170 (= 65% -- 13 small units)
Apples New -- 7 units - 70 (=35% -- 7 small units

Before
--------
Orange -- Draw 3 units
Apples -- Draw 7 units

Apples -- Shade out 10 in each of the 7 units (to represent 70 apples sold)
Orange -- Shade out 10 in each of the 3 units, then add 170 oranges. You get 3 small units + 3 x 10 + 170 = 13 small units

10 small units --> 3 x 10 + 170 = 200

1 small unit --> 200 / 10 = 20

Apples --> 20 x 7 + 70 = 210 apples.