Q&A - PSLE Math

[underthesea]

Can help me to solve this question, ST Nick 2008 P6 SA1.


There are some rubber balls and plastic balls in a bag. If 1 rubber ball were to be removed from the bag, then 1/7 of the remaining balls in the bag would be rubber balls. If 2 plastic balls were to be removed from the bag instead of 1 rubber ball, then 1/5 of the balls left behind would be rubber balls. If another 5 rubber balls were to be put into the bag, what fraction of all the balls would be rubber balls?

TIA.

[Guan Hui]

underthesea i hope you understand my working=D


rubber: Total
1u+1: 7u+1 <<< no balls taken out because I add the 1 ball back.
after taking out 2 plastics balls, no of rubber remains. no. of total -2.
new ratio
1u+1:7u-1
but we also know that the fraction after taking out 2 plastic balls is 1/5 so…
1u+1:5u+5

From here we can actually see 7u-1 and 5u+5 is the same thing.
So… 5u+5=7u-1
u=3
no. of rubber balls = u+1=4
total no of balls= 7u+1=22

Checking…
when take out 1 rubber ball
no of rubber balls= 4-1=3
total no of balls= 22-1=21
fraction= 3/21=1/7(correct)

when take out 2 plastic balls
no of rubber balls= 4 unchanged
total no of balls = 22-2=20
fraction= 4/20=1/5(correct)

Since now it is correct, we need to complete the question.
add 5 rubber balls.
no of rubber balls =4+5=9
total no of balls= 22+5=27
fraction= 9/27=1/3

The ans is 1/3=D

hope it is understandable=D

[Kiasee888]

Hi guys. If you are interested in getting tuition for your son or daughter for a primary school subject like maths and want to know how tutors calculate their rates then visit this site: http://www.tuition1on1.com/subjectrates.htm


They also have a http://www.tuition1on1.com/apps/links/ that is still under construction. They told me that they will be posting up free exam papers very soon.

I joined as a tuition coordinator for primary school subjects with them. If anyone interested in getting a tutor, you can pm me or if you are interested in joining as a coordinator, also can. :lovesite:
http://www.postimage.org/

[Maths Monster]

delete

[underthesea]

Thanks Guan Hui and Maths Monster for your reply.


Here’s another problem I can’t solve. Need your help.

A class has 44 pupils. 3/4 of the boys and 2/5 of the girls are short-sighted. There are 26 boys and girls short-sighted. How many more boys than girls in the class.

[Vanilla Cake]

underthesea:

A class has 44 pupils. 3/4 of the boys and 2/5 of the girls are short-sighted. There are 26 boys and girls short-sighted. How many more boys than girls in the class.

Hi underthesea,

I am the younger sister of Vanilla Cake and your question is similar to this question posted in another forum.

Hogwarts photography club has 1150 members.20% of the men and 25% of the women are non-professional photographers. A total of 250 members are non-professional photographers. How many more men than women are there in the club?

There are 2 different solutions given and let's wait for Guan Hui and Maths Monster to come up with preferably model solutions.

Solution 1
3/4 Boys + 2/5 Girls = 26 (Short-sighted)
Multiply by 5/2,
15/8 Boys + Girls = 65

Boys + Girls = 44

7/8 Boys =21
Boys = 24
Girls = 44-24=20

Difference = 24-20 = 4
There are 4 more boys than girls in the class.

Solution 2
Boys
S : N
3 : 1

Girls
S : N
2 : 3

S: Short-sighted
N: Normal vision

Since units for boys are not the same as units for girls, let's use u for Boys' units and p for Girls' units.

3u+2p = 26 (short-sighted)
1u+3p = 18 (normal vision) - *
* - (Multiply by 3)
3u+9p = 54

9p-2p = 54-26
7p = 28
p = 4

Girls: 5p = 5x4 = 20
Boys: 44-20 = 24

Difference = 24-20 = 4
There are 4 more boys than girls in the class.

Thank you.

[Maths Monster]

delete

[underthesea]

Hi,


Thanks for your reply. If, say the child doesn't know how to do this question is exam. Given that the total number of pupils is not big, can the child use Guess and Check method to solve this problem?

We know the number of boys and girls must be a whole number and the number of boys is a factor of 4 while the number of girls is a factor of 5.

So, for boys => 4, 8, 12, 16, 20, 24, 28, 32, 36, 40
for girls = > 5, 10, 15, 20, 25, 30, 35, 40

From the above, we can figure out 24 + 20 = 44. Hence, there are 4 more boys than girls.

Do you think marks will be awarded? :?

[skyjuice]

Does anyone have any comment or feedback on Fabian Ng’s Math classes? I’m considering sending my son as he’s rather weak and it’s PSLE yr for him. A friend said he’s good for her son but each 1.5hr class is $75. With such fees, I would like to know more of his classes if anyone cares to share. Thanks!!

[Maths Monster]

delete