Q&A - PSLE Math

[pixiedust]

Hi all, I am completely :stupid: with CA1 revision.

I would greatly appreciate help with the following :

Q1:
Thanks iFruit for the solution. Yesterday there was a parents' briefing in school and it was re-iterated that algebra method, although accepted in PSLE, is not strongly encouraged. I would like to check if there are any other way to solve this problem withOUT using alegbra or ratio(not taught yet) :

Nancy has 3 times as many buttons as Mary.
Every day, Mary uses 5 buttons and Nancy uses 4 buttons.
When Mary has finished using all her buttons, Nancy has 88 buttons left.
(1) How many days does Mary take to use all her buttons ?
(2) How many buttons are there altogether at first ?


Q2:
We have the answer and solution for this question but I need the systematic way to approach such question so that if the number changes, we can still find the solution. Basically :
(a) how to find the number of pairings ?
(b) how to determine if there is a middle no partner number ? how to find it ?

10+12+14+16+.......+46+48+50=

Q3:
There were 20 questions in a test. 5 marks given for each correct answer, 2 marks deducted for each wrong answer. No mark for each blank answer. Mary scored 74 marks. How many questions did she answer correctly ?

I did it this way :
Gap --> Each wrong question, -7marks.
26 marks lost --> closest guess, 3 wrong questions hence 21 marks lost.
From here, I use G&C to find number of questions Mary answered correctly. It is easy enough due to the small numbers in the question.
Is there a more efficient way instead of G&C ?

[youngtay]

Hi, please help


Mrs Goh sells mooncakes in boxes of twos and fours. At first, there were four times as many small boxes as bog boxes. After selling half the number of small boxes and some big boxes, Mrs Goh packs 5 more boxes of mooncakes. Given that there are thrice as many small boxes as bIg boxes and there is a total of 300 mooncakes in the unsold boxes, find the number of big boxes sold. (Ans given 20 - A* maths)


TIA

[pixiedust]

I use algebra method.

Anyone can contribute model/branching or other PSLE methods ?

During school open house, Pri 5 sold 55 litres of cranberry juice. The cranberry juice was sold in cups containing 200 ml & 300 ml. An equal number of cups containing 200ml & 300mil were sold. how many cups of cranberry juice did Pri 5 sell?

Algebra method :

Let the number of cups be u
200u + 300u = 55000
500u=55000
u=110

Ans: Pri 5 sold 220 cups of cranberry juice.

check :
110 x 200ml +
110 x 300ml
--> 55 litres

[tianzhu]

pixiedust:

Q3:
There were 20 questions in a test. 5 marks given for each correct answer, 2 marks deducted for each wrong answer. No mark for each blank answer. Mary scored 74 marks. How many questions did she answer correctly ?

I did it this way :
Gap --> Each wrong question, -7marks.
26 marks lost --> closest guess, 3 wrong questions hence 21 marks lost.
From here, I use G&C to find number of questions Mary answered correctly. It is easy enough due to the small numbers in the question.
Is there a more efficient way instead of G&C ?

Hi

Suppose he answered all 20 questions correctly.

20*5 = 100 marks
100 – 74 = 26

5+2 =7 marks lost for 1 wrong answer.

5 marks lost for 1 unanswered question.

26 ----- (21+5) ------3 wrong questions+ 1 unanswered question.

Therefore, 3 wrong questions and 1 left unanswered.

He answered 16 questions correctly

Best wishes

[NickleBee Tutors]

JSSK:

Hi,

Can anyone help with this question? Thanks.

During school open house, Pri 5 sold 55 litres of cranberry juice. The cranberry juice was sold in cups containing 200 ml & 300 ml. An equal number of cups containing 200ml & 300mil were sold. how many cups of cranberry juice did Pri 5 sell?

Regards

This is pretty straightforward and can be solved with some understanding of the context of the question. The thought process stated below is definitely easier to understand (than algebra) for most upper primary children.

The child has to realize that since an equal number of 200ml and 300ml cups were each sold, that means that for every one 200ml cup, there was also another 300ml cup sold.

So if you draw a diagram, you can draw a big circle to represent 1 group with one 200ml cup plus one 300ml cup inside.

The million dollar question would then be \"How many groups were there?\"

Now each group would have 200ml + 300ml = 500ml in total.

Thus number of groups = 55000ml / 500 ml = 110 groups

Since 1 group contains one 200ml cup plus one 300ml cup inside,

Thus there would be 110 x 2 = 220 cups in all.

QED

Yours
Zhou Shicai
NickleBee Tutors

[Amberz]

The total of 4 numbers P, Q, R,and S was 291.

When P was tripled, Q was halved, R was increased by 20 and S was decreased by 25, the four numbers became equal.
Find the value of R. :?

[iFruit]

pixiedust:

Hi all, I am completely :stupid: with CA1 revision.

I would greatly appreciate help with the following :

Q2:
We have the answer and solution for this question but I need the systematic way to approach such question so that if the number changes, we can still find the solution. Basically :
(a) how to find the number of pairings ?
(b) how to determine if there is a middle no partner number ? how to find it ?

I think it'd be useful to remember the following formula ( and be able to apply, of course).

nth term = 1st term + (n-1)d where n = number of terms, d = common diff.

Sum of n terms = (1st term + last term)n/2

Once you find n, you can just find the middle term from n/2, but usually it is not necessary if one remembers the above formulae.

Just my POV.

[iFruit]

Amberz:

The total of 4 numbers P, Q, R,and S was 291.

When P was tripled, Q was halved, R was increased by 20 and S was decreased by 25, the four numbers became equal.
Find the value of R. :?

This is a backwards problem..

Assume at the end, each number became 3u.

Then at the beginning:

P = 3u/3 = u
Q = 3u x 2 = 6u
R = 3u - 20
S = 3u + 25

u+6u+3u-20 + 3u + 25 = 291

13u = 286--->u =22

R = 3u - 20 = 46

HTH.

[tianzhu]

youngtay:

Hi, please help


Mrs Goh sells mooncakes in boxes of twos and fours. At first, there were four times as many small boxes as bog boxes. After selling half the number of small boxes and some big boxes, Mrs Goh packs 5 more boxes of mooncakes. Given that there are thrice as many small boxes as bIg boxes and there is a total of 300 mooncakes in the unsold boxes, find the number of big boxes sold. (Ans given 20 - A* maths)


TIA

Hi youngtay

Due to time constraints, I’ll give you some pointers instead of MD.

In answering this question, a student should recognise that the before and after ratios are of different kinds. The before ratios represent number of boxes of moon cakes and the after ratios represent value or the number of moon cakes.

You may use MD or UM to solve it.

In the end, here we are talking about number of unsold moon cakes.

Small boxes ------ 6 units
Big boxes ------- 4 units

10 units ------- 300
1 unit -------30

Number of unsold moon cakes in small boxes --------6*30 -------180
Number of unsold small boxes of moon cakes --------180/2 ------90
Number of unsold moon cakes in big boxes --------4*30 -------120
Number of unsold big boxes of moon cakes --------120/4 ------30

At first, here we are talking about number of boxes.

Number of moon cakes in small boxes ---------90*2 --------180
Number of moon cakes in big boxes ---------180*4 --------45

Next, find the number of big boxes sold.

30 - 5 ----- 25
45 – 25 -----20

Best wishes

[anneshirleygilbert]

tianzhu:

anneshirleygilbert:

With the use of the calculator, I took about 9 minutes to get to arrive at the answer. Is this too long to spend on a 4 or 5 marks question in the PSLE? Pls advise.

Hi anneshirleygilbert

I am sorry; I am not in a position to answer your question this time.

I believe the school teachers would have taught the students time management.

Your kid’s teacher would be in a better position to advise you.

Perhaps, you may find something interesting in this article.

http://www.epopular.com.sg/my-kid/100-for-maths-yes-you-can.php

Best wishes

Thank you for the link, tianzhu. You are so resourceful. The articles are enlightening and helpful.