Q&A - PSLE Math

[tianzhu]

Thank you for your help.


RGPS SA2 2008 Q43.


http://www.postimage.org/

[joyceling]

tianzhu:

Thank you for your help.


RGPS SA2 2008 Q43.


http://www.postimage.org/

the answer is 2cm.

[Happygolucky]

HI Tianzhu,


If I’m not wrong , one of the cubes at the uppermost layer was not in the question.

Is the answer 2 cm?

[joyceling]

cubes for each level is (top to bottom ) 1,3,6

water difference between top 2 levels and bottom level is the cubes difference: 8 cubes=1120-352x3=64, so each cube is 8 cm3
so the height is 2cm

[tianzhu]

Happygolucky:

HI Tianzhu,


If I'm not wrong , one of the cubes at the uppermost layer was not in the question.

Is the answer 2 cm?

Hi

This question is from http://www.misskoh.com
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I guess one of the cube is not in the question as it looks different from the others.

[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.

[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.