Q&A - PSLE Math

[youngmum]

For the above question 1 - for step 1 why 12 + 24? I don't understand the first step, the rest ok. Pls explain to me. Thanks.


i solved it the ratio way though.

13: 8 = 39 : 24
1:8 = 15:120

So, 39 x 2 = 78
24/2 = 12

total = 78 + 12 = 90

[ChiefKiasu]

Priskhipo:

HI Sarah,


The 3 questions share similar strategy.
...

Priskhipo, thanks for the detailed \"workout\"!

[heutistmeintag]

Priskhipo, may I know if you are a teacher or tuition teacher? Your answers are so professional. Do you take students?

[lantian]

http://farm4.static.flickr.com/3252/2887152891_1a46c5c3de_o.jpg\">

[Priskhipo]

heutistmeintag:

Priskhipo, may I know if you are a teacher or tuition teacher? Your answers are so professional. Do you take students?

Hi heutistmeintag,

Thanks you. I'm a teacher but not in the mainstream kind.
Sorry, I'm not taking in any students for tuition.
I'm glad to be able to help out in this website and continue to do so if time permit.

[Priskhipo]

youngmum:

For the above question 1 - for step 1 why 12 + 24? I don't understand the first step, the rest ok. Pls explain to me. Thanks.


i solved it the ratio way though.

13: 8 = 39 : 24
1:8 = 15:120

So, 39 x 2 = 78
24/2 = 12

total = 78 + 12 = 90

Compare the orange bar (13 small units) to the bar consists of blue rectange (1 small unit) with green and white dotted-line rectangles.

The difference is indicated by the arrow (12 units).
The green rectangle represents 12 number and the white dotted-line rectangle represents 24 number.

Therefore 12 units represent 12 + 24 = 36

[lantian]

Sarah:

Q2. A, B and C are three numbers. The average a, B and C is 5 greater tha the averaga of A and b. If C is 38, find the total of A, B and C.

http://farm4.static.flickr.com/3010/2898264072_99252aab49_o.jpg\">

[ChiefKiasu]

lantian:

http://farm4.static.flickr.com/3010/2898264072_99252aab49_o.jpg\">

Sigh... I'm still trying to learn to think like you

[corneyAmber]

Sorry to differ a little here with lantian. A small error but big in concept.


Technically speaking(since this is math and precision is impt hence its concept), it is not correct to name the first 2 boxes as A and B in each of the peach box. Can only put the 2 boxes together as
(A+B) to be reflecting the question more accurately. The concept is to know that (A+B) = 2 units of its average and not A=B=1 unit each.

Example:
A=3, B=5, then A+B = 3 + 5 = 8. Avg = 8/2 = 4.
this implies A+B = 2*4 = 8 but does not imply that A=4 and B=4.

Even though the answer is correct, the concept of A=B=1 unit is a wrong assumption to get to the answer.

It might just be an oversight when lantian drew the diagram but thought I should point it out that potentially the working cud be marked wrong for concept.

[ChiefKiasu]

ks2me:

...Technically speaking(since this is math and precision is impt hence its concept), it is not correct to name the first 2 boxes as A and B in each of the peach box. Can only put the 2 boxes together as

(A+B) to be reflecting the question more accurately. The concept is to know that (A+B) = 2 units of its average and not A=B=1 unit each...

Yes, the labelling is not correct, but lantian did color all the similar boxes with the same color, so I guess that's his intention, anyway. Thanks for clarifying!