Q&A - PSLE Math

[tianzhu]

ozora:

thanks for the clear explanation

Hi

You're welcome.

Best wishes

[is_me_lblue]

Hi,


Question as follow :

The ratio of the area of A to B is 1:2. If 30% of B overlaps with A, what percentage of the whole figure is not overlapped ?

Thk u

Anwer given is 75%.

[Belle2011]

is_me_lblue:

Hi,


Question as follow :
The ratio of the area of A to B is 1:2. If 30% of B overlaps with A, what percentage of the whole figure is not overlapped ?
Thk u
Anwer given is 75%.

I try to explain as detailed as possble because I do not know how to insert a diagram which speaks a thousand words.
B is a bigger circle as compared to A.
The overlapping area is 30% of B and thus put 3 in that overlapping area and the remaining of B will be 7. Since A:B is 1:2, the total parts for A will be 5 and since the overlapped area is 3, the not overlapped of A is 2.

Total parts = 7+3+2 = 12.
Total Not Overlapped parts = 7+2 = 9.

9/12 = 75%.

Cheers,
Belle.

[ADoc]

ozora:

Adoc thanks for the pictorial explanation. Must really get use of using this approach to teach the children.

cheers! Hope it's useful..

[ADoc]

Belle2011:

is_me_lblue:

Hi,


Question as follow :
The ratio of the area of A to B is 1:2. If 30% of B overlaps with A, what percentage of the whole figure is not overlapped ?
Thk u
Anwer given is 75%.

I try to explain as detailed as possble because I do not know how to insert a diagram which speaks a thousand words.
B is a bigger circle as compared to A.
The overlapping area is 30% of B and thus put 3 in that overlapping area and the remaining of B will be 7. Since A:B is 1:2, the total parts for A will be 5 and since the overlapped area is 3, the not overlapped of A is 2.

Total parts = 7+3+2 = 12.
Total Not Overlapped parts = 7+2 = 9.

9/12 = 75%.

Cheers,
Belle.

Hi is_me_Iblue!
just a \"rough\" diagram. Hope this is useful for your visualisation.

cheers!
ADoc
http://postimage.org/image/312yr9mpw/

[Emelyn]

Hi,


Brain jammed....cannot solve the problem. Sign of old age too....
Asking for help here .......


http://postimage.org/image/2kyiwxjj8/

In the figure above, which is not drawn to scale, 1/4 of the circle is shaded. The ratio of the area of the square to the area of the rectangle and circle is 5:7. 40% of the square is shaded. 1/3 of the rectangle is shaed. What is the ratio of the area of the circle to the square to the rectangle ?

MTIA !!

[pixiedust]

Appreciate help with the following question.


My solution is using listing multiples method.
Is there a faster way?
Such questions, analysing the gap (like the oranges question) doesn't work?

Some NCC cadets are gathered in a hall.
If they are grouped into 7 cadets in a team, there will be 4 cadets left out.
If they are grouped into 9 cadets in a team, there will be 6 cadets left out.
How many cadets are there in the hall if the number of cadets is between 20 and 70 ?

[tianzhu]

Emelyn:

Hi,


Brain jammed....cannot solve the problem. Sign of old age too....
Asking for help here .......


http://postimage.org/image/2kyiwxjj8/

In the figure above, which is not drawn to scale, 1/4 of the circle is shaded. The ratio of the area of the square to the area of the rectangle and circle is 5:7. 40% of the square is shaded. 1/3 of the rectangle is shaed. What is the ratio of the area of the circle to the square to the rectangle ?

MTIA !!

Hi

In this question, the units are in pretty nice numbers.

You may use Units Method or MD.

Circle
Shaded ----- 1 unit
Unshaded ------ 3 units
Total -------4 units

Rectangle
Shaded ----- 1 unit
Unshaded ------ 2 units
Total ------- 3 units

Square
Shaded ----- 2 unit
Unshaded ------ 3 units
Total --------5 units

C:S:R ------4:5:3

Now you may wish to try this one.

Best wishes

http://farm3.static.flickr.com/2407/2673911274_a92537168a_o.jpg\">

[tianzhu]

pixiedust:

Appreciate help with the following question.

My solution is using listing multiples method.
Is there a faster way?
Such questions, analysing the gap (like the oranges question) doesn't work?

Hi

Systematic listing of multiples is commonly used by primary students to solve this question.

Best wishes

[tianzhu]

pixiedust:

My solution is using listing multiples method.
Is there a faster way?
Such questions, analysing the gap (like the oranges question) doesn't work?

Hi

Good Afternoon.

Is there a faster way? Is there a more efficient method to make life a little easier for our young ones?

I gave your question some thoughts last night.

This question is usually solved with “Systematic Listing” .It takes a bit of time but it also ensures that students gain some marks as long as they don’t make careless mistakes.

I am not too sure whether working it this way is more efficient than SL, do take a look and decide if it works for you.

First the thought process, we are given a number of cadets between 20 and 70.

So we have a big group of NCC cadets in a hall.

For case 1, they are grouped into 7 cadets in a team, this means multiples of 7.As for groups of 9, it also means multiples of 9.

For comparison, we have some groups of 7 plus 4 in case 1 and some groups of 9 plus 6 in case 2.Remember the total number of cadets in the hall is the same in both cases.

Hence, it’s likely we would have more groups of 7 than groups of 9.The difference between 6 and 4 is 2.Therefore, we need to find the multiples of 7 which is greater than the multiples of 9 by 2.

The closest multiples of 7 and 9 to 70 is 63.The difference between them is zero.

Next move down, (8*7) ------ 56 and (6*9) -------54.This gives a difference of 2.

The answer is (56+4) ------60 or (54+6) -------60

For my solution, I’ve drawn more MD than necessary to show the thought process. Once you become more familiar, you may do away with some of them.

First, find the difference between the remaining cadets after the groupings. Next proceed to find the multiples which give the required difference and you are on the way to the answer.

Hope you’ll find it useful.

Best wishes

http://farm6.static.flickr.com/5171/5511424062_c0bfacc8c2_b.jpg\">