Q&A - PSLE Math

[tianzhu]

Hi

Another two questions on two changed quantities types of questions.

1) A box contained 50cent coins and 20 ct coins in the ratio of 2:3. When I took out four 50ct coins, exchanged them for 20ct coins, and then put the money back into the box, the ratio became 2:7. Find the sum of money in the box. (9.60)

2) The number of grey marbles to black marbles in a bowl was in the ratio of 4:5. Later, 8 grey marbles were taken out and 20 black marbles were added into the bowl. After that, the ratio of grey marbles to black marbles became 4:11. How many marbles of each colour were in the bowl in the end? (20, 55)

Best wishes

[Brenda10]

tianzhu:

Hi

Another two questions on two changed quantities types of questions.

1) A box contained 50cent coins and 20 ct coins in the ratio of 2:3. When I took out four 50ct coins, exchanged them for 20ct coins, and then put the money back into the box, the ratio became 2:7. Find the sum of money in the box. (9.60)

2) The number of grey marbles to black marbles in a bowl was in the ratio of 4:5. Later, 8 grey marbles were taken out and 20 black marbles were added into the bowl. After that, the ratio of grey marbles to black marbles became 4:11. How many marbles of each colour were in the bowl in the end? (20, 55)

Best wishes

Hi tianzhu

For Q1, We have to consider 50 Cents (-4) and 20 cents (+10) when comparing the value. That would be able to work out and get the total of $9.60.

For Q2, it is quite straight forward.

Thank you for providing us all these sample.

[atutor2001]

Vanilla Cake:

tianzhu:

[quote]Well,ratio problems are supposed to be solved by ratio.I love using ratio but i'm always stuck at challenging questions that involve ratio.One quick(inaccurate) example is this: The marbles that A and B has is in the ratio 6:7. Then,124 marbles were added to the total and the ratio became 2:3.How many marbles were there at first? These kind of questions are common and its almost impossible to solve using model. If your dd knows how to use ratio to solve such challenging problems,then your dd should be alright if she wants to use model now as she still have the \"skills\" to apply the ratio method if she needs it.However,if she cannot solve using ratio,then she may face difficulty with the challenging questions.

Hi

I picked up this quote from the thread, Tips for improving in PSLE. It was from a member called livewith_vanilla.

I think the question she posed is a pretty interesting one, it involves two changed quantities, but the number(124) involved in the transaction is lumped into one lot instead of two different portions.

The marbles that A and B has is in the ratio 6:7. Then,124 marbles were added to the total and the ratio became 2:3.How many marbles were there at first?

Have fun.
Best wishes

Hi Uncle Tianzhu,
There seems to be more than one possible correct answer for this question. I have posted a reply to \"livewith_vanilla\" to seek her clarification for this \"One quick(inaccurate) example is this:.....\"[/quote]I remember seeing this question in Andrew Er's assessment book.

It is true mathematically that if we have only 1 equation with 2 unknowns, then the number of answers can be infinite. However, this particular question will have only one answer because of the following reasons:
(a) the number of marbles must be \"whole number\"
(b) the numbers in the questions are all prime numbers : 13; 5 and 124=2x2x31

This question was intended to test a student's understanding that if a certain amount is divided into 13 U and it becomes 6 W after adding 124 to 13U, we can convert the 6 W to U by cutting each W into 3 parts. There is no other way of cutting such that 124 can be divided without remainder.

[tianzhu]

Brenda10:

Thank you for providing us all these sample.

Hi Brenda10
You're welcome.

You’re swift and accurate.
If you are a P6 student, you can certainly look forward to an illuminating performance in PSLE 2010.

Best wishes

[Brenda10]

tianzhu:

Brenda10:

Thank you for providing us all these sample.

Hi Brenda10
You're welcome.

You’re swift and accurate.
If you are a P6 student, you can certainly look forward to an illuminating performance in PSLE 2010.

Best wishes

Hi tianzhu

Thank you.

In fact I am actually very blur on all these ratio questions. :? However I feel is important for my dd (P5) to get the fundamental right and we have spent a few days try to learn/understand the different approach on ratio questions. Hopefully by more practice, dd can gradually handle better when the questions getting more complicating.

[tianzhu]

Hi Brenda10


A P5 girl, that’s even more impressive considering the fact that she has just started to learn ratios.

Wishing you and your daughter success in your journey towards A* for PSLE Maths in 2011.

Best wishes

[CoffeeCat]

atutor2001:

Vanilla Cake:

[quote=\"tianzhu\"]
Hi

I picked up this quote from the thread, Tips for improving in PSLE. It was from a member called livewith_vanilla.

I think the question she posed is a pretty interesting one, it involves two changed quantities, but the number(124) involved in the transaction is lumped into one lot instead of two different portions.

The marbles that A and B has is in the ratio 6:7. Then,124 marbles were added to the total and the ratio became 2:3.How many marbles were there at first?

Have fun.
Best wishes

Hi Uncle Tianzhu,
There seems to be more than one possible correct answer for this question. I have posted a reply to \"livewith_vanilla\" to seek her clarification for this \"One quick(inaccurate) example is this:.....\"

I remember seeing this question in Andrew Er's assessment book.

It is true mathematically that if we have only 1 equation with 2 unknowns, then the number of answers can be infinite. However, this particular question will have only one answer because of the following reasons:
(a) the number of marbles must be \"whole number\"
(b) the numbers in the questions are all prime numbers : 13; 5 and 124=2x2x31

This question was intended to test a student's understanding that if a certain amount is divided into 13 U and it becomes 6 W after adding 124 to 13U, we can convert the 6 W to U by cutting each W into 3 parts. There is no other way of cutting such that 124 can be divided without remainder.[/quote]To assure those who got multiple answers that they are not seeing stars...
I used the old fashioned algebra way ,
i let the marbles at first be 6x and 7x.
and the marbles added to A and B be y and 124-y respectively
using the last ratio, i form the equation
3 (6x+y) = 2( 7x+ 124-y)
i get this diophantine equation 4x + 5y = 248
the smallest solution is form when x=2, and y =48. (26 marbles at first)
the next solution is when x= 2+5 = 7 and y = 48-4 = 44 (91 marbles at first).
I stopped here but i suspect additional solutions can be obtained by adding 5 to x, and subtracting 4 from y.
It's quite late so i will digest what you said tmr morning, so it's possible i might be reading something wrongly.

[Vanilla Cake]

CoffeeCat:

atutor2001:

[quote=\"Vanilla Cake\"]
Hi Uncle Tianzhu,
There seems to be more than one possible correct answer for this question. I have posted a reply to \"livewith_vanilla\" to seek her clarification for this \"One quick(inaccurate) example is this:.....\"

I remember seeing this question in Andrew Er's assessment book.

It is true mathematically that if we have only 1 equation with 2 unknowns, then the number of answers can be infinite. However, this particular question will have only one answer because of the following reasons:
(a) the number of marbles must be \"whole number\"
(b) the numbers in the questions are all prime numbers : 13; 5 and 124=2x2x31

This question was intended to test a student's understanding that if a certain amount is divided into 13 U and it becomes 6 W after adding 124 to 13U, we can convert the 6 W to U by cutting each W into 3 parts. There is no other way of cutting such that 124 can be divided without remainder.

To assure those who got multiple answers that they are not seeing stars...
I used the old fashioned algebra way ,
i let the marbles at first be 6x and 7x.
and the marbles added to A and B be y and 124-y respectively
using the last ratio, i form the equation
3 (6x+y) = 2( 7x+ 124-y)
i get this diophantine equation 4x + 5y = 248
the smallest solution is form when x=2, and y =48. (26 marbles at first)
the next solution is when x= 2+5 = 7 and y = 48-4 = 44 (91 marbles at first).
I stopped here but i suspect additional solutions can be obtained by adding 5 to x, and subtracting 4 from y.
It's quite late so i will digest what you said tmr morning, so it's possible i might be reading something wrongly.[/quote]Hi All,
I am VC's mum and this is the http://www.kiasuparents.com/kiasu/forum/viewtopic.php?t=11112&start=20 from thread starter - livewith_vanilla:

Ermmm...sorry but that's not the question i have actually.I actually just thought of some random one.It is not actually a question.Its just an example,something that i just thought of randomly without any thought.Yes,the answers will be vague.I will take an example from my school paper tmr and post it here as an accurate question with only one answer.Sorry!

Hi atutor2001,
I believe that this question is invalid and will not likely to appear in any upcoming PSLE Maths exams. If such example does appear in Andrew Er's assessment books, pls let us know which assessment book? Pls elaborate more on your 2 reasons that this particular question has only one answer instead of multiple answers.
Thanks.

[Brenda10]

Hi tianzhu


Thank you for your encouragement.

There is still a long way to go and hopefully we can keep up our stamina.

We do hope that the effort will be pay off.

Have a nice day.

[trytry]

2008

http://www.jamesangtutors.com/forum/viewtopic.php?p=5520&sid=dd197883788c1e3bdaea959288b33365

7. Mrs Smith bought some mangoes and mangosteens for $16.50. She bought 5 fewer mangoes than mangosteens. Each mango cost $1.80 more than the mangosteen. How many mangosteens did she buy?

Anyone has solution to the above question?
Crack my head can't solve.