Q&A - PSLE Math

[tianzhu]

atutor2001:

My answer is $380

Hi
I think you are right.
I wrongly assume the sevenths to be on the remaining sum.

Best wishes

[Muffins]

tianzhu:

Hi Muffins


When 40% more lorries from an industrial estate entered carpark P ----- how do we interpret this statement?

If the intention of the question setter is that the number of existing lorries is increased by 40%, I agree with your solution.

Best wishes

To tell you the truth, Tianzhu, I interpreted the problem exactly like you did at first! However, I think the question setter is trying to say 40% of the units of the lorries at first, 3 units, or as I put it in my working step, 15 units. If you work out the problem by multiplying the units of lorries at first by 140%, the problem would work out. By the way, the answer for the question is 564. Also, if you want a more detailed working of this problem I'd be happy to do it for you!

Wishing your child(ren) all the best for their PSLE!

[Praise]

Hi,


Can someone help to solve the following questions :-

1) There are some black and white marbles in a bag. If 20 black marbles are removed from the bag, the total number of marbles left will be 7 times the number of black marbles left. If 50 white marbles are removed from the bag, the total number of marbles left will be 5 times the number of black marbles left. How many marbles are there in the bag?

2) 65% of the animals on a farm were cows and the rest were goats. When 240 more cows and goats were added to the farm, the percentage of cows increased by 20% and the number of goats doubled. How many goats were there on the farm at first.

3) The ratio of the number of pencils to the number of erasers in a box was 2 : 3. When 42 pencils were added and 15 erasers were removed, the ratio of the number of pencils to erasers became 3:4. How many erasers were there left in the box?

[tianzhu]

Hi Muffins


Thank you for your offer.I understand your solution.

You have assumed that the unit of measures for both ratios is the same.

I appreciate your well wishes.May all PSLE kids be abundantly blessed with fruitful results.

Best wishes

[atutor2001]

tianzhu:

I wrongly assume the sevenths to be on the remaining sum.

Best wishes

Thanks for confirming. I find it amazing that such questions are set for P6

[youngmum]

Dharma:

elkniwt:

[quote=\"tianzhu\"]Thank you for your help.


http://farm3.static.flickr.com/2547/3984945203_a6bc525a6c_o.jpg\">

Hi,

Is the answer 19:14?

If yes, this is how I got it, but I'm not sure if you can do the working this way in primary sch.

Unshaded = Area of triangle PRV + Area of triangle VSQ - overlap
= 1/2 x QR x PV + 1/2 x QR x VQ - 75
= 1/2 x 45 x 22 - 75
= 420
Shaded = Area of rect - unshaded
= 45 x 22 - 420
= 570
Hence ratio of shaded to unshaded is 570:420 = 19:14

The unshaded area = Area of 1/2 rect - 75 = 495 - 75 = 420
Shaded area : Unshaded area = (495 + 75) : 420 = 570 : 420 = 19:14[/quote] how the unshaded area = Area of 1/2 rect
i cannot see it

[small]

youngmum:

how the unshaded area = Area of 1/2 rect

i cannot see it

Hi youngmum,

Please invert the diagram and take a look, it will be more clear.

The base of two triangles is 45 cm and the height is 22cm, thus their area:

1/2 x 45 x 22 which will be same as half of rectangle (area of rectangle = 45 x 22 ).

[youngmum]

small:

Please invert the diagram and take a look, it will be more clear.
The base of two triangles is 45 cm and the height is 22cm, thus their area:
1/2 x 45 x 22 which will be same as half of rectangle (area of rectangle = 45 x 22 )

i cannot see it

[small]

Hi youngmum,


From our working, we knew the area for the two triangles is 1/2 x (45x22)

Let the area of big rectangle be 1 unit,which is 1 unit = (45 x 22)

Half rectangle =
= 1/2 x 1 unit
or
= 1/2 of ( 45x22 )

Hope this clear.


Don't worry too much on the method of solution, the most important thing is our child able to solve the question during the exam, it just that correct working will save some of their time which will allow them to have more time for other questions.

[Vanilla Cake]

Praise:

1) There are some black and white marbles in a bag. If 20 black marbles are removed from the bag, the total number of marbles left will be 7 times the number of black marbles left. If 50 white marbles are removed from the bag, the total number of marbles left will be 5 times the number of black marbles left. How many marbles are there in the bag?


2) 65% of the animals on a farm were cows and the rest were goats. When 240 more cows and goats were added to the farm, the percentage of cows increased by 20% and the number of goats doubled. How many goats were there on the farm at first.

3) The ratio of the number of pencils to the number of erasers in a box was 2 : 3. When 42 pencils were added and 15 erasers were removed, the ratio of the number of pencils to erasers became 3:4. How many erasers were there left in the box?

Quick one before I go to school. Wish every P6 student all the best in his/her PSLE 2009.

A1
Assume black be B and white be W.
Case 1
(B-20)/W = 1/6
W=6B-120
W+120=6B
1/3W +40=2B

Case 2
B/(W-50)=1/4
W-50=4B
1/2W-25 = 2B

1/3W+40=1/2W-25
1/2W-1/3W=65
1/6W=65
W=390
B=[1/2(390)-25]/2 = 85

Number of marbles in the bag is 390+85 = 475

A2
Before
Cows-> 65%
Goats->35%

After
Cows->120/100x 65% = 78%
Goats->35x2 = 70%

78-65=13
13+35=48
48%=240
1% = 5
35% = 175
Number of goats on the farm at first was 175

A3
P : E
2:3 (Before)

(2 units+42)/(3 units-15) = 3/4
9 units - 45 = 8 units + 168
1 unit = 213
3 units = 639
639-15=624
Number of erasers left in the box was 624