Q&A - PSLE Math

[ozora]

Got this question:

The ratio of the number of pupils in class A to the number of pupils in class B is 5: 4 at first. 1/5 of the pupils in class A and 7/8 of those in class B were girls. After reshuffling of pupils between 2 classes, the number of boys in each class is equal to the number of girls. If the total of 24 pupils are affected, how many pupils are there in each class at first?
I got the answer class A-42 and B-30 which are wrong.
Anyone can help?
Answer is class A-40 and B-32

[MathIzzzFun]

ozora:

Got this question:

The ratio of the number of pupils in class A to the number of pupils in class B is 5: 4 at first. 1/5 of the pupils in class A and 7/8 of those in class B were girls. After reshuffling of pupils between 2 classes, the number of boys in each class is equal to the number of girls. If the total of 24 pupils are affected, how many pupils are there in each class at first?
I got the answer class A-42 and B-30 which are wrong.
Anyone can help?
Answer is class A-40 and B-32

Hi

a model approach

http://www.flickr.com/photos/62167097@N02/5834499397/in/photostream

cheers.

[corneyAmber]

http://i54.tinypic.com/xprbkl.jpg\">

[P5G]

Please help.


Nan Hua
http://www.orlesson.org/orp/10Ma/2010-P6-Math-CA1-NanHua.pdf
Question 23
Rick and his brother shared 2 pizzas. Rick ate twice as much as his brother. What fraction of the pizzas did his brother eat?

Answer given is: 1/3


St Nicholas
http://www.orlesson.org/orp/10Ma/2010-P6-Math-CA1-StNic.pdf
Question 5
Nomee distributed 7/8 of a log cake equally among her twin sister and 2 brothers. What fraction of the log cake did each of them receive?

(1) 4 3/8
(2) 3 1/2
(3) 7/24
(4) 7/32

Answer given is (3) 7/24.


Please advise if the answer for St Nicholas should be 1/3 instead of 7/24?

TIA.

[tianzhu]

Angel25:

12. \tAt 09 00 Jonathan drove from Kuala Lumpur to Singapore. He first drove to Malacca which is 200 km away at an average speed of 100 km/h. When he reached Malacca, he rested for 45 minutes. Then, he continued driving the remaining of his journey to Singapore at an average speed of 90 km/h.

At what time did he reach Singapore?

Hi

There may be some missing information.

Any information on a)The distance between KL and Singapore or b) The distance between Malacca and Singapore.

Best wishes

[tianzhu]

Hi P5G


Rick ate twice as much as his brother.
Divide a pizza into 3 parts.
2 parts ----- Rick
1 part ------ His brother
His brother ate 1/3 of a pizza


7/8 of a log cake is distributed to 1 twin sister and 2 brothers. The three of them shared equal portions.
7/8 ÷ 3 or 7/24.

Best wishes

[P5G]

tianzhu:

Hi P5G


Rick ate twice as much as his brother.
Divide a pizza into 3 parts.
2 parts ----- Rick
1 part ------ His brother
His brother ate 1/3 of a pizza


7/8 of a log cake is distributed to 1 twin sister and 2 brothers. The three of them shared equal portions.
7/8 ÷ 3 or 7/24.

Best wishes

Uncle Tianzhu,

Why do you multiply by 7/8 for the log cake but not by 2 for the pizzas?
How do we decide when to multiply by the number given and when not to?
Your help will be greatly appreciated.

TIA.

[tianzhu]

Hi P5G


The question asks a student to find out what fraction did Rick’s brother eat.

Rick ate twice as much as his brother.
Therefore, Rick ----- 2 units and his brother ----- 1 unit.

You may just work on one pizza
Divide a pizza into 3 parts.
Rick ----- 2 parts
His brother ------1 part

His brother ------ 1/3 of a pizza

If you choose to work with 2 pizzas.
One pizza ----- 3 parts, therefore 2 pizzas -------6 parts
Rick ----- 4parts
His brother ------2 parts

The answer is 2/6 which is also 1/3

Best wishes

[tianzhu]

Hi


Imagine a piece of log cake cut into 8 big pieces.

Out of these 8 pieces, 7 pieces are distributed to 1 twin sister and 2 brothers.

Further subdivide these 7 pieces into 21 smaller pieces to be shared among the 3 people.

Hence each one of them (1 twin sister and 2 brothers ) receives 7 smaller pieces each.

There are a total of 24 smaller pieces in the beginning.(The whole log cake)

Hence, 7/24

Best wishes

[P5G]

tianzhu:

Hi


Imagine a piece of log cake cut into 8 big pieces.

Out of these 8 pieces, 7 pieces are distributed to 1 twin sister and 2 brothers.

Further subdivide these 7 pieces into 21 smaller pieces to be shared among the 3 people.

Hence each one of them (1 twin sister and 2 brothers ) receives 7 smaller pieces each.

There are a total of 24 smaller pieces in the beginning.(The whole log cake)

Hence, 7/24

Best wishes

Uncle Tianzhu,

Thank you for your clear explanations. So for the log cake, we are talking about a 'whole' log cake and not just 7/8.

Will the answer be just 1/3 if \"of a\" is removed from the question as follows:
Nomee distributed 7/8 log cake equally among her twin sister and 2 brothers. What fraction of the log cake did each of them receive?

TIA.