Q&A - PSLE Math

[youngmum]

Sorry people - i must be a really lousy mother. i still don't understans pls help. For this question,


(1) There are 1000 lockers in a school with 1000 students. The first student opens all the lockers. The second student closes lockers 2,4,6,8,10 till 1000. The third student changes the state (close opened ones and open closed ones) of lockers 3,6,9,12 till 1000 . The fourth student changes the state of lockers 4,8,12,16 till 1000. How many lockers are opened in the end?

Number of cycles ---> 1000 divided by 12 = 83 r 4

83 cycles ---> 83 x 7 = 581
remaining 4 ---> 2
581 + 2 = 583

i don't understand the part that why you need to mulitply 83 by 7???

(2) For the container question - Container A contains 250 red marbles and 200 blue marbles. Container B contains 600 red marbles and 150 blue marbles. How many red and blue marbles must be moved from container A to container B such that 25% of the marbles in container A and 75% of marbles in container B are red?


I don't understand the 1u 25% and 1 part 75%. What is the difference between the unit and part?


(3) Lily is meeting a friend at a certain time. If she drives at 80 km/h, she will be 1/3 hour late. If she drives at 60 km/h, she will be 3/4 hour late. How long will the journey take if she drives at 90 km/h?

Did you get 10/9 hours?

Distance is constant
ratio of speed 4 : 3 -- i know this is 80:60 = 4:3
ratio of time 3 : 4

I don't understand how did you get the ratio of time?


(4) 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?


Apples ----- 7u - 10 = 7 parts
Oranges --- 3u + 170 = 13 parts

Why sometimes parts and sometimes unit??????????????????????? I just don't understand... ... pls help.



How can my son pass maths i really woner. even the mother can't solve all these questions. :((

[Dharma]

Hi youngmum,


Hope the slides below helps
http://www.postimage.org/
http://www.postimage.org/

[Dharma]

youngmum,


For your 2nd Qn,

1 unit used to represent the no. of red marbles transfered from Container A to Container B and 1 part used to represent the transfer of no. of blue marbles from Container A to Container B

If u are not comfortable w/ units and parts, u can use X and Y.

The 25% (Red) is the end result for Container A. Given in Qn. So, blue must be 75% for Container A.

[Dharma]

youngmum,


If distance is fixed, as given in Qn

The ratio of time is inversely proportional to the ratio of speed

Speed = 4:3
Time = 3:4

Distance = Speed x Time

D (fixed) = Speed (Increase) x Time (decrease) …and vice versa

[youngmum]

Thanks Dharma, i am still trying to figure it out. Really thanks so much. Sometimes i really wonder how can children 12 year old brain think so much.

[Dharma]

hi youngmum,


Qn 1 is not so easy a an average 12 yr old. Non-routine type qn …looks more like a maths olympiad type.

Forgot your Qn 4 earlier.

I used units and parts to represent proportion of apples and oranges b4 and after Mr Tan’s transactions respectively.

U can use anything else u like to represent the proportions…but need to understand that the proportions are not the same.

[Drdj]

Hi there,


I have a question from Red Swastika Prelim 2008 Paper.
The answer seems to give the area of the grille (784 cm2), but length of iron needed to make the grille should be the circumference, and length. Am I correct?


http://www.postimage.org/

[Drdj]

Drdj:

Hi there,


I have a question from Red Swastika Prelim 2008 Paper.
The answer seems to give the area of the grille (784 cm2), but length of iron needed to make the grille should be the circumference, and length. Am I correct?


http://www.postimage.org/

http://www.postimage.org/image.php?v=Pq1KGN99

[tianzhu]

Drdj:

Drdj:

Hi there,

I have a question from Red Swastika Prelim 2008 Paper.
The answer seems to give the area of the grille (784 cm2), but length of iron needed to make the grille should be the circumference, and length. Am I correct?

Hi
I think what you wanted is just to check the answer.It is 344cm.

[enoawng]

James Ang:

enoawng:

Can any help?


There were 186 adults in an auditorium. When 7/12 of the men and 4/9 of the women left the auditorium, the total number of adults who remained became 90. How many men were there in the auditorium at first?

let me try;

12/12 - 7/12 = 5/12 of men left behind
9/9 - 4/9 = 5/9 of women left behind

5/12 of men and 5/9 of women = 90
1/12 of men and 1/9 of women = 18
9/12 of men and 9/9 of men = 18x9 = 162
3/12 of men = 186 -162 = 24
12/12 of men = 24 x 4 = 96 men.

There are 96 men in the auditorium at first.

Thanks. However I think one step is missing for clarity, right?
9/12 of men and 9/9 of WOmen = 18x9 = 162 .........(1)
12/12 of men + 9/9 of WOmen = 186 .....................(2)
Subtracting (2) from (1)
3/12 of men = 186-162=24

Cheer