Q&A - PSLE Math

[tianzhu]

charsen:

Help needed for this question.

A class of 40 pupils took part in a test.Each pupil was given the same number of questions to answer in the test.5 marks were awarded for each correct answer.3 marks were deducted for each wrong answer.No marks was awarded or deducted for any question not attempted.
For every 21 questions,2 questions were not attempted and 7 questions were answered wrongly.The class scored a total of 6240 marks for the test.How many questions were there on the test paper?

Hi

First consider a set of 21 questions.

2 wrong questions ------ nil marks

7 wrong questions ------- 7*3 -------- 21 marks deducted

12 correct answers -------- 12*5 ------- 60 added

Hence 60 -21 ------39 marks for a set of 21 questions.

6240/39 -------- 160

Number of questions -------- 160*21 ------- 3360

Number of questions per testpaper --------- 3360/40 ------- 84

Best wishes

[tianzhu]

Winx5015:

Please help !!!

(1). In an enrichment class of 48 students, each student must take at least a language class or a mathematics class. 7/12 of the students choose French language and 1/4 of the students choose French language and a mathematics class. How many students choose only a mathematics class?

Hi

Please take a look at the solution for a similar type of question.

Best wishes

http://farm8.staticflickr.com/7243/6966119100_e47f2d8176_z.jpg\">

[tianzhu]

Winx5015:

(4). Mr Ramy walks from his house to his office every morning. If he walks at a speed of 65m/min, he will be late for work by 5 minutes. If he increases his speed by 15m/min, he will be early for work by 4 minutes. How far is his office from his house?

Hi

65*5 -------- 325

80*4 ------- 320

325 + 320 ------- 645

645/15 ------- 43 min

Distance ------- 43*65 + 325 -------- 3120m or 43*80 – 320 ------- 3120m

Best wishes

[tianzhu]

Winx5015:

(3). There were 180 apples and 240 oranges in Box A. There were 400 apples and 120 oranges in Box B. How many apples and how many oranges must be transferred from Box B to Box A such that 50% of the fruits in Box A and 3/4 of the fruits in Box B are apples?

Hi

There are many examples of similar types of questions in this thread.

There are a few approaches, using pictorial way, Units and Parts or letters of the alphabet.

Here’s an example using MD, you may also use U and P to replace the boxes and circles.

Best wishes

http://farm8.staticflickr.com/7191/6924699990_b871018c47_z.jpg\">

[tianzhu]

Hi


Another similar question to your Q3.

There are two bag of stones labeled A and B. In bag A ,there are 350 black stones and 500 white stones. In bag B , there are 400 black stone and 100 white stones. How many black and white should be transfer from bag B to bag A so that 50% of the stones in bag A and 75%of those in bag B are black?

There are different approaches; you may solve it pictorially, by units and parts or letters of the alphabet.

Number of black stones ------ 350 + 400 ------ 750

Number of white stones ------- 500 + 100 ------ 600

Bag A

Black: White ------ 1u:1u (50% black, 50% white)

Bag B

Black: White ------ 3p:1p (75% black, 25% white)

2p -------- 750 – 600 ------150

1p ------- 75

3p ------- 225

1p -------- 75

Number of black stones transferred out ------ 400 – 225 ------- 175

Number of white stones transferred out ------ 100 – 75 ------- 25

Best wishes

[tianzhu]

Winx5015:

(2). Mrs Tan bought 3kg of strawberries and 8kg of grapes. Mrs Chin bought 3kg of grapes and 8kg of strawberries. Mrs Chin spent $35 more than Mrs Tan. If 1kg of grapes cost $8, how much would 1kg of strawberries cost?

Hi

You may use MD.

5 kg of strawberries ------- 35 more than 5 kg of grapes

Hence, 1kg of strawberries is $7 more expensive than 1kg of grapes.

1 kg of strawberries -------- 8+7 ------15

Best wishes

[jewelbox]

Hi, how do I solve this qn? Thanks. http://i47.tinypic.com/2j1si1k.jpg\">

[nounou]

This Q comes from Pei Chun Prelim 2012 Q18.

I would like to know if there is another method to find answer for b and c.
One that is easier to understand and remember. Please help. :?: :imdrowning:
:thankyou: :thankyou: :thankyou:


quote=\"charsen\"]

ms_lim:

Can help to explain how to solve this question?

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C)
I try to find the general rule and i found it for how to find the total no.of tiles.It is (pattern no.x2-1)x(pattern no.x2-1)+4.
I subtracted 4 from the total no. of tiles in each question.For example,29-4=25.
You know that 25 is a square root of 5,which is a odd no.So,if you subtract 4 from all the total no. of tiles,you get the square root of odd no.Next,you should find the odd no. of the square root you found.
take pattern 3 as an example.
29-4=25
5x5=25
i need to find the relation ship between the pattern no. and the odd no.So,I used the formula to find odd numbers which is 2n-1-->(3+3)-1=5
And there,i found the formula.
Cheers[/quote]

[Winx5015]

thanks tianzhu

[tianzhu]

Treelodge:

This Q comes from Pei Chun Prelim 2012 Q18.

I would like to know if there is another method to find answer for b and c.

Hi

This question is difficult as compared to the other pattern questions in previous papers.

I don't have much time to prepare slide, so just give you some pointers.

Please bear with it if you find it disorganised.

For the white tiles, notice that the number of white tiles in the odd pattern number is always 4 more than the number of white tiles in the even pattern number. That is to say when the number of white tiles in the even section changes, the number of white tiles in the next odd section increases by 4.

Take for example 8 to 12, 32 to 36.

So we focus on the even pattern number.

8 -------- 4*2 ------- 2*2*2

32 ------- 16*2 -------4*4*2

72 -------- 36*2 -------6*6*2

So we can see that the it follow a pattern of (n*n*2) where n is the pattern number.

For the odd pattern number, simply add 4 to the number tiles before it.

For pattern 30, we have 30*30*2 -------- 1800.

For the total number of tiles, take note that he difference between 2 numbers are 8, 16,32,24,32,40,48 …….

Hence, we have 8(1+2+3+4+5+6……..)

Students should be familiar with calculating the sum of numbers from 1 to 100.

We follow a pattern of 5 + 8 * (sum of numbers from 1 to n-1) , where n is the pattern number.

In this case, pattern 86.

So we have n ------ 86 and (n-1) ------ 85

Therefore, we need 5 + 8 * ( sum of numbers from 1 to 85)

The total number ------- 5 + 8( 85*86)/2 -------- 29245

Best wishes