Q&A - PSLE Math

[cycjca]

Hi all

I’m new here, dd1 taking her PSLE in less than 2 weeks time and too bad only found this place very recently.

In anycase, hope someone can show this hopeless mama how to solve some of these questions:

1) Mrs Chua has just enough money to buy 4 kg of crabs & 3 kg of prawns. However she bought 2 kg of crabs & 4 kg of prawns & had $4.50 left. If she exchanged the 3 kg of crabs with 3 kg of prawns, how much money would she have left?

2)Mr Tan had 200 apples and oranges.
1/3 of the apples and 1/4 of the oranges were rotten. He threw away 60 rotten fruits altogether. How many rotten oranges were there?

Have been waitg for her teacher to go thru the questions with them…but nothing as yet! so panicking. Thanks in advance!

[octopusmum]

Hi all

need to ask for solutions for these 2 questions from Rosyth Prelim 2010 paper:

1) A tour bus which left Kuala Lumpur at 10.30am was scheduled to reach Singapore by 5pm. After travelling for 2 hours at its usual speed, the bus stopped for 30 minutes due to some problems. In order to reach Singapore punctually, the bus increased its speed by 8.5km/h. Given that the bus took 4 hours to complete the rest of the journey, find the distance between Kuala Lumpur and Singapore.

2) Xiaver, Yati and Zul each had a certain number of stamps. At first, Yati had 200 stamps more than Xiaver and Zul had 3/4 the number of stamps Yati had. After Yati gave away 1/8 of her stamps to Xiaver, she had 60 fewer stamps than Xiaver. What was the ratio of the number of stamps Xiaver had to the number of stamps Yati had to the number of stamps Zul had at first? Give your answer to the simplest form

thks v much

[Vanilla Cake]

octopusmum:

Hi all

need to ask for solutions for these 2 questions from Rosyth Prelim 2010 paper:

Hi octopusmum,
Due to time constraint (as I need to go to sleep), pls refer to the solutions done by Uncle Tianzhu and Uncle Observer.

octopusmum:

1) A tour bus which left Kuala Lumpur at 10.30am was scheduled to reach Singapore by 5pm. After travelling for 2 hours at its usual speed, the bus stopped for 30 minutes due to some problems. In order to reach Singapore punctually, the bus increased its speed by 8.5km/h. Given that the bus took 4 hours to complete the rest of the journey, find the distance between Kuala Lumpur and Singapore.

Solution by Uncle Tianzhu
Original time from KL to Singapore ----- 6.5h\t\t\t\t\t\t
\t\t\t\t\t\t
Consider the second half of journey\t\t\t\t\t\t
\t\t\t\t\t\t
Time ------- 4: 4.5 ------- 8:9\t\t\t\t\t\t
Speed -------9:8\t\t\t\t\t\t
\t\t\t\t\t\t
1 unit -----8.5km/h\t\t\t\t\t\t
8 units ------ 68 km/h\t\t\t\t\t\t
\t\t\t\t\t\t
Distance between Kuala Lumpur and Singapore ------68*6.5 ----- 442km\t

OR

http://psle2010a.blogspot.com/2010/09/speed-p6-2010-rosyth-p2-q17.html

octopusmum:

2) Xiaver, Yati and Zul each had a certain number of stamps. At first, Yati had 200 stamps more than Xiaver and Zul had 3/4 the number of stamps Yati had. After Yati gave away 1/8 of her stamps to Xiaver, she had 60 fewer stamps than Xiaver. What was the ratio of the number of stamps Xiaver had to the number of stamps Yati had to the number of stamps Zul had at first? Give your answer to the simplest form

Solution by Uncle Tianzhu
At First\t\t
X ------8units\t\t
Y ------- 8 units+200\t\t
Z -------3/4 of Y\t\t
\t\t
In the End\t\t
X -------9 units + 25\t\t
Y ------7 units + 175\t\t
Z remains the same\t\t
\t\t
7units+175+60 ----- 9 units +25\t\t
1 unit ------105\t\t
\t\t
X ------- 105*8 ------ 840\t\t
Y -------1040\t\t
Z -------780 (3/4 of Y)\t\t
\t\t
Therefore X:Y:Z ---------- 42:52:39

OR

http://psle2010a.blogspot.com/2010/09/fraction-p6-2010-sa2-rosyth-p2-q18.html

OR

http://www.onsponge.com/forum/35-thinkingmathonsponge/4868-prelim-2010-rosyth-working-backwards.html.

[iFruit]

cycjca:

Hi all

I'm new here, dd1 taking her PSLE in less than 2 weeks time and too bad only found this place very recently.

In anycase, hope someone can show this hopeless mama how to solve some of these questions:

1) Mrs Chua has just enough money to buy 4 kg of crabs & 3 kg of prawns. However she bought 2 kg of crabs & 4 kg of prawns & had $4.50 left. If she exchanged the 3 kg of crabs with 3 kg of prawns, how much money would she have left?

This problem is not well defined and can have any number of solutions. For example, we can set cost of prawns/kg = cost of crabs/kg = $4.5 and come up with a final answer of $4.5

or you can set cost of crabs = $10/kg, prawns = $15.50/kg and come up with an answer of $21

[iFruit]

cycjca:

2)Mr Tan had 200 apples and oranges.

1/3 of the apples and 1/4 of the oranges were rotten. He threw away 60 rotten fruits altogether. How many rotten oranges were there?

Have been waitg for her teacher to go thru the questions with them..but nothing as yet! so panicking. Thanks in advance!

Let Apples = 12u
Oranges = 200 - 12u

Rotten apples = 1/3 x 12u = 4u
Rotten Oranges = 1/4 x (200-12u) = 50 - 3u

Total rotten = 4u + (50 - 3u) = 60 ---> u = 10

Rotten oranges = 50 -3u = 50 - 30 = 20

[tianzhu]

cycjca:

1) Mrs Chua has just enough money to buy 4 kg of crabs & 3 kg of prawns. However she bought 2 kg of crabs & 4 kg of prawns & had $4.50 left. If she exchanged the 3 kg of crabs with 3 kg of prawns, how much money would she have left?

Hi

Please check figures highlighted in blue.Is it 3kg or 2kg?

Best wishes

[tianzhu]

cycjca:

1) Mrs Chua has just enough money to buy 4 kg of crabs & 3 kg of prawns. However she bought 2 kg of crabs & 4 kg of prawns & had $4.50 left. If she exchanged the 3 kg of crabs with 3 kg of prawns, how much money would she have left?

If 2kg of crabs is changed to 3kg of crabs, here's the solution.

Hope this helps.

Best wishes

http://farm5.static.flickr.com/4133/5019157330_02c85ba4c0_b.jpg\">

[tianzhu]

Hi


Hope this helps.

Best wishes.

http://farm5.static.flickr.com/4147/5019157380_593d6e70b1_b.jpg\">

[Yu Xuan]

cycjca:

1) Mrs Chua has just enough money to buy 4 kg of crabs & 3 kg of prawns. However she bought 2 kg of crabs & 4 kg of prawns & had $4.50 left. If she exchanged the 3 kg of crabs with 3 kg of prawns, how much money would she have left?

Using the numbers as Tianzhu
This is how I did it;

4C + 3P = 0 (amt of money left)
3C + 4P = 4.50 (amt of money left)

Comparing the two types of spending above;
For every 1C he exchange for 1P, he will also get $4.50 left, i.e
1C = 1P + 4.50
3C = 3P + 13.50

Hence,
3P + 4P = $4.50 + $13.50 = $18

[Yu Xuan]

cycjca:

2)Mr Tan had 200 apples and oranges.
1/3 of the apples and 1/4 of the oranges were rotten. He threw away 60 rotten fruits altogether. How many rotten oranges were there?

R denotes orange

1 A + 1 R= 200
1/3 A +1/4 R = 60

multiply the second equation by 3 so that we can have 1 whole of A.
1A + 3/4R = 180

Comparing this equation with the first equation;
1/4 R --> 200 - 180 = 20