Q&A - PSLE Math

[tianzhu]

htn:

tianzhu
how to use MD to solve this q? can teach me?
TIA

Hi mumwgals and htn

MD refers to “Model Drawing” or Model Diagram”.

In this question, the units or boxes showing boys and girls in the parade ground and indoor hall are of different measures. They are not of the same sizes.

A student should note that the number of boys and girls remains the same before and after the transfer. The total number of 1000 did not change at all.

There are two unknowns with two linear equations.Recall the time at lower primary when you’re using the stars and moons, you could actually solve the equations pictorially as shown in the slide. This question is crafted in such a way that it is pretty neat to use MD.

Otherwise, students may choose to work using “Units and Parts” or letters of the alphabet as representation. Use the method you are comfortable with.

Hope this helps.

Best wishes

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[pensiveowl]

tianzhu:

pensiveowl:

Hi.. could someone pls help ans this question - thanks


Mr Lim had some red and green beads. 60% of his beads were red and the rest were green. He lost some of his red beads and the percentage of the beads which were red decreased by 40%. He then had 760 red and green beads left. How many red beads did he lose?
(1) 190
(2) 240
(3) 285
(4) 360

Hi

I believe you just want to double check the answer to this MCQ.

It is 240.

Best wishes

Yes this is the given ans tianzhu - I posted the question hoping to find out how to do it though

[smileplease]

tianzhu:

smileplease:

Colin had $80 more than Donovan at first. After Colin spent 75% of his money and Donovan spent 40% of his money, Donovan had $50 more than Colin. How much money did Colin have at first?

Hi

You may use UM or MD to solve it.

I use 20units instead of 10units for the common bar (Colin and Donovan) because I want to avoid working with decimals.

At first.
Colin -------- 20 units + 80
Colin ------- 20 units

In the end
Colin ------- 5 units + 20
Donovan ------ 12 units

Donovan had $50 more than Colin.
12 units ------ 5 units + 20 + 50
1 unit ------- 10

Colin ------20*10 -------200 at first.

Best wishes

The answer given by the teacher is 280. I just simply do not understand teacher's working.

[tianzhu]

smileplease:

The answer given by the teacher is 280. I just simply do not understand teacher's working.

Hi

I am sorry to mix up the names.

Colin ------(20*10) + 80 -------280 at first.

I'll revise the solution.

Best wishes

[MathIzzzFun]

smileplease:

tianzhu:

[quote=\"smileplease\"]
Colin had $80 more than Donovan at first. After Colin spent 75% of his money and Donovan spent 40% of his money, Donovan had $50 more than Colin. How much money did Colin have at first?

Hi

You may use UM or MD to solve it.

I use 20units instead of 10units for the common bar (Colin and Donovan) because I want to avoid working with decimals.

At first.
Colin -------- 20 units + 80
Colin ------- 20 units

In the end
Colin ------- 5 units + 20
Donovan ------ 12 units

Donovan had $50 more than Colin.
12 units ------ 5 units + 20 + 50
1 unit ------- 10

Colin ------20*10 -------200 at first.

Best wishes

The answer given by the teacher is 280. I just simply do not understand teacher's working.[/quote]hi

answer is 280..if you follow tianzhu's workings, you will get 280 ... Colin had 20 * 10 + 80 = 280.

cheers.

[MathIzzzFun]

pensiveowl:

tianzhu:

[quote=\"pensiveowl\"]Hi.. could someone pls help ans this question - thanks


Mr Lim had some red and green beads. 60% of his beads were red and the rest were green. He lost some of his red beads and the percentage of the beads which were red decreased by 40%. He then had 760 red and green beads left. How many red beads did he lose?
(1) 190
(2) 240
(3) 285
(4) 360

Hi

I believe you just want to double check the answer to this MCQ.

It is 240.

Best wishes

Yes this is the given ans tianzhu - I posted the question hoping to find out how to do it though[/quote]hi
Blue beads as % of initial total = 40%
Red beads left as % of initial total = 60% x 60% = 36%
% left = 76% of initial total = 760, so 1% = 10
Red beads lost = 40% x 60 % = 24% = 24 x 10 = 240 beads.

cheers.

[tianzhu]

pensiveowl:

Yes this is the given ans tianzhu - I posted the question hoping to find out how to do it though

Hi

Red ----- 6 units
Green ----- 4 units

Decreased by 40%
Red -------- 6/10 *6 ------3.6 units

3.6 + 4 ------7.6
7.6 units -------760

1 unit ------100

In the end, red ------360

Therefore lose 600-360----240

Best wishes

[jeysri72]

Hi,


Can anyone give a clear explanation on how to solve the problem in RGS Prelim Maths 2011 Paper 2 Question 18?
The question is: Beaker A and Beaker B contain some water. If 46.5 ml of water is drained out from Beaker A, the volume of the water in beaker A will be 60% that of the water in Beaker B. If 32.5 ml of water is drained out from Beaker B will be 85% that of the water in Beaker A. What is the total volume of water in Beaker A and Beaker B?
Ans:290.5 ml

[pensiveowl]

tianzhu:

pensiveowl:

Yes this is the given ans tianzhu - I posted the question hoping to find out how to do it though

Hi

Red ----- 6 units
Green ----- 4 units

Decreased by 40%
Red -------- 6/10 *6 ------3.6 units

3.6 + 4 ------7.6
7.6 units -------760

1 unit ------100

In the end, red ------360

Therefore lose 600-360----240

Best wishes

Ok thanks very much again..

[jeysri72]

Can anyone give an explanation and solution for henry park 2011 prelim maths.

Q17:there were 1000 students at the parade ground and indoor sports hall. 40% of the 600 students at the parade ground were girls. 60% of the students at the indoor sports hall were girls. After some of the students in both venues moved from one venue to the other, 30% of the students in the parade ground and 70% of the students at the indoor sports hall were girls. How many students were there in the indoor sports hall after the movement had taken place? Ans: 450
Q18: A large warehouse had a total of 2180 black, red and white t-shirts for sale. the ratio of black t shirt to red t shirts was 3:1. after 2/5 of the black t shirts and 1/3 of the white t shirts and none of the red t shirts were sold, there were 1504 t shirts left. how many red t shirts were there at first? ans: 380