Q&A - PSLE Math

[tianzhu]

Hi


This is a question on Compound Proportion.

I think it’s covered in secondary Maths. You may wish to check with the school teacher if it is in the syllabus.

The trick to solve such question is to tackle one variable at a time.

One way to look at it.

3 men took 4 days to paint 3 houses.
1 man took 4 days to paint 3/3 or 1 house.
4 men took 1 day to paint 1 house.
4 men took 9 days to paint 9 houses.

Best wishes

[tianzhu]

Hi Brenda10 and Belle2011


You’re welcome.

Best wishes

[Lynn2]

Hi Brenda10,


That is an interesting way to solve the math prob.do u mind sharing with me the name of the book too?thanks

Lynn2

Belle2011:

Brenda10:

Hi Tianzhu

DD learn from one of the book that we got. This book has similar example to the question we encountered. Its come with model but by following the example she did not draw the model. Have PM for you.

Thank you

Dear Brenda10,
Can you share the same of this book?
I cannot solve these problems at all. Maybe my IQ is just too low. I often cannot handle the last few (4 or 5 marks) questions.
Thank-you.
Cheers,
Belle.

[Hifive]

Hi


Source : Andrew Er’s New Syllabus Mathematics

A class has a total of 40 pupils. Each girl has an average mass of 40kg and each boy has an average mass of 48kg. If the total mass of the girls is 512 kg lighter than the total mass of the boys, how many girls are there in the class ?

Ans: 16 girls.

Though the book provided the solution to the question but don’t quite get it and hope that parents/students here can provide alternative method of solution.

Thanks & Regards

[Brenda10]

Hifive:

Hi


Source : Andrew Er's New Syllabus Mathematics

A class has a total of 40 pupils. Each girl has an average mass of 40kg and each boy has an average mass of 48kg. If the total mass of the girls is 512 kg lighter than the total mass of the boys, how many girls are there in the class ?

Ans: 16 girls.

Though the book provided the solution to the question but don't quite get it and hope that parents/students here can provide alternative method of solution.

Thanks & Regards

Hi Hifive, how about using the guess and check method.


Boy Girl

20 x 48=960, 20 x 40 = 800 (Diff:160 - wrong)
22 x 48 = 1056, 18 x 40 = 720 (Diff: 336) - wrong)
24 x 48 = 1152, 16 x 40 = 640 (Diff: 512- Correct)

There are 16 girls in the class.

Thank you

[tianzhu]

Hi


Hope this helps.

Best wishes.

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[Belle2011]

Can anybody please help to solve:


In a primary school, 50 pupils were surveyed on the pets they liked. 80% of the pupils liked dogs, 50% of them liked birds and 34% of them liked rabbits. The survey results were that 1 out of every 5 pupils did not like any of the animals.
What is the least number of pupils who liked all 3 types of animals.
(model ans : 2).

Thanks.

Cheers,
Belle.

[Hifive]

Hi Brenda10 and tianzhu


Thank you for providing the alternative methods of solution. Yes, make it easier to understand now. THANKS SO MUCH!

Sorry, 2 more to go from the same book:

1. Benny spent 15% of his salary on food and $440 on camera. Then he spent 30% of the remainder on clothes and saved the rest of his salary. He found that the amount he spent on clothes was $120 more than the amount he had spent on food.
(a) How much did he spend on food? Ans: $360
(b) How much did he save? Ans: $1120


2. Wilson purchased a certain number of wallets at $6 each. If he had purchased those at $8 each, he would have purchased 20 fewer wallets. How much money did he have?
Ans : $480

Thanks again.

[tianzhu]

Hi Hifive


I'll take Q2 first, it's an interesting one.

One way to look at it to to see the relationship between the difference in number of wallets and the amount of money.

Hope this helps.

Best wishes.

http://farm6.static.flickr.com/5287/5305519560_ea03d6bc22_b.jpg\">

[Brenda10]

Hifive:

Hi Brenda10 and tianzhu


Thank you for providing the alternative methods of solution. Yes, make it easier to understand now. THANKS SO MUCH!

Sorry, 2 more to go from the same book:

2. Wilson purchased a certain number of wallets at $6 each. If he had purchased those at $8 each, he would have purchased 20 fewer wallets. How much money did he have?
Ans : $480

Thanks again.

If we use the guess and check method, then the common factor of 6 and 8 are 24, 48.

$6, $8
240/6= 40, 240/8 = 30 (diff 10 bags - wrong)
480/6= 80, 480/8 = 60 (diff 20 bags - correct)

As for the other question dd tries to solve but unable to get the answer. Hopefully Tianzhu and somebody else can help too.

Hi Hifve, would you mind to show the methods provided by the book so as we can also learn another approaching to this question.

Thank you