Q&A - PSLE Math

[tianzhu]

Brenda10:

DD said she had done this question.

1/3 - 1/5 = 2/15
70-10 = 60
60/2= 30
30 x 15 = 450

Hi Brenda10

Work backwards, and you'll realise your answer of 450 is wrong.

My suggestion is to draw a MD as it'll give you a clearer picture.

Best wishes

[Brenda10]

mrswongtuition:

I think this method forgot to take into consideration that though in the 2nd scenario Adam gave lesser to Rahman (by 2/15), it does not mean that 70 - 10 = 2/15 of what Adam has. Because the difference was GIVEN to Rahman and part of what Rahman has now, not totally lost of given away to others.


I'm not too sure how to explain in words, but take a look at the model I've posted and compare from there

Hi mrswongtution

Thank you for your highlgiht.

Will feedback to dd.

[Brenda10]

tianzhu:

Brenda10:

DD said she had done this question.

1/3 - 1/5 = 2/15
70-10 = 60
60/2= 30
30 x 15 = 450

Hi Brenda10

Work backwards, and you'll realise your answer of 450 is wrong.

My suggestion is to draw a MD as it'll give you a clearer picture.

Best wishes

Hi Tianzhu

Thank you for your suggestion.

Will ask dd to verify the worksheet once she got it back.

[mrswongtuition]

tianzhu:

mrswongtuition:

I think the teacher got 450 because....


From this onwards:
BASED ON SAME TOTAL for Adam (3 x 5 = total 15 units),
We can change the units into....

If Adam gave 1/3 of his share to Rahmad...
All units multiply by 5 to get a total of 15 units at first:

A [-][-][-][-][-][-][-][-][-][-]
R [-][-][-][-][-][-][-][-][-][-][ 70 ]

If Adam gave 1/5 of his share to Rahmad....
All units multiply by 3 to get a total of 15 units at first:

A [-][-][-][-][-][-][-][-][-][-][-][-]
R [-][-][-][-][-][-][-][-][-][-][-][-][ 10]

Comparing the above 2 models, the teacher compared as 2 units = 70 - 10 = 60
2 units = 60/2 = 30
15 units = 20 x 15 = 450

Regards,
Mrs Wong
Tuition Web: http://www.mwtuition.com.sg
Personal Blog: thelittleonesinmylife.blogspot.com

Hi Mrs Wong

I think your first solution is correct.

The answer is 225.

Best wishes

Hi, I showed the 2nd answer based on the mistake I think happened when the teacher worked out and got 450 as answer

Quite common mistake that students make, that's why I typed out the model to make it clearer

[worridaddy]

Hi can anyone help :

When Larry's age was twice Henry's age, Don's age was 27. When Don's age was twice Larry's age, Henry's age was 15. What was Henry's age when Don was 58 years old ?
Thanks ! :?

[atutor2001]

worridaddy:

Hi can anyone help :

When Larry's age was twice Henry's age, Don's age was 27. When Don's age was twice Larry's age, Henry's age was 15. What was Henry's age when Don was 58 years old ?
Thanks ! :?

This question use model very confusing.
Use units with \"difference in age never change\" and simultaneous, is easier.
So the easy way is :

L... H... D
2u. u... 27
v.. 15.. 2V
.... ? ... 58

Difference in age never change :
2u-u = v-15 that is u = v-15 ...... Eqn (1)
u-27 = 15-2v that is u = 42-2v ......Eqn (2)

Solving (1) & (2) :
v-15 = 42-2v
3v = 57
v = 19
2v = 38

58-38 = 20
15+20 = 35 (answer)

Easier if Pr kids learn simult eqns - most of them can

If die die must draw model, then draw comparison models for

u+15 is same length as v .....1st comparison model
u+2v is same length as 42....2nd comparison model

then replace the \"2v\" in the 2nd model with 2u+30

[worridaddy]

Thanks atutor2001.

[atutor2001]

worridaddy:

Thanks atutor2001.

You are most welcome, strongly recommend to teach simult eqns to pr school students. It makes solving problem sum much easier.

[chrisu]

Thanks to all for responding to this post. My daughter came back yesterday evening and told me that the teacher and the Maths HOD agreed to accept both 225 and 450 as answer since the question is ambiguous.


The answer is 225 if comparison is made after the 1/3 and 1/5 is given to Rahmad. The answer will be 450 if the comparison is made before the 1/3 and 1/5 is given to Rahmad.

As for proving of Adam = 450;
If Adam = 450, Rahmad will be 370.
If Adam gives 1/3 of 450 to Rahmad,
Rahmad will get : 370 + 150 = 520.
520 - 450 = 70 (which is 70 more than Adam before the 1/3 is given out).

If Adam gives 1/5 of 450 to Rahmad,
Rahmad will get : 370 + 90 = 460.
460 - 450 = 10 (which is 10 more than Adam).

[tianzhu]

chrisu:

Thanks to all for responding to this post. My daughter came back yesterday evening and told me that the teacher and the Maths HOD agreed to accept both 225 and 450 as answer since the question is ambiguous.


The answer is 225 if comparison is made after the 1/3 and 1/5 is given to Rahmad. The answer will be 450 if the comparison is made before the 1/3 and 1/5 is given to Rahmad.

As for proving of Adam = 450;
If Adam = 450, Rahmad will be 370.
If Adam gives 1/3 of 450 to Rahmad,
Rahmad will get : 370 + 150 = 520.
520 - 450 = 70 (which is 70 more than Adam before the 1/3 is given out).

If Adam gives 1/5 of 450 to Rahmad,
Rahmad will get : 370 + 90 = 460.
460 - 450 = 10 (which is 10 more than Adam).

Hi

Consider this in real life situation.

If I have $450 and I give $150 away, I’ll have $300 left.

I find it difficult to understand the explanation from the school.

Best wishes