Q&A - PSLE Math

[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

[LiteracyICare]

Hi mrswongtuition


Thank you for the link! Appreciate the help there too! I’ll certainly take a look at the resources.

Much gratitude!

[chrisu]

Agreed and that's why it took me one whole night to understand the answer 450 is compared with before the share was given out. Hope that such question will not be out in the PSLE. Too time consuming even to draw the models.


Maybe the school should inform the publisher that there are 2 answers to the question.

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

[tianzhu]

chrisu:

Agreed and that's why it took me one whole night to understand the answer 450 is compared with before the share was given out. Hope that such question will not be out in the PSLE. Too time consuming even to draw the models.

Hi

The question is why are we comparing with the original amount?

How about this question?

If Mark gives Lenny 14 stickers, he will have the same number of stickers as Lenny. If Lenny gives 10 stickers to Mark, the ratio of the number of stickers Lenny has to that of Mark will be 2: 5. How many stickers does Mark have?

Best wishes