Q&A - P5 Math

[kkme]

Thks Tianzu!

tianzhu:

kkme:

2) When Kok Hwa's age was twice Bala's age, Mike's age was 30. When Mike's age was twice Kok Hwa's age, Bala was 21. Given that Mike was the oldest among the three persons, find Bala's age when Mike was 62 years old.

Hi

For Q2, due to time constraints, I'll provide some pointers instead of drawing MD.

Case 1
Bala ----- 1 unit
KH ----- 2 units
Mike ------ 30

Difference in age between KH and Bala ----- 1 unit

Difference in age between Mike and KH ----- 30 – 2 units

Case 2
Bala ----- 21
KH ----- 21 + 1 unit
Mike ------ (21 + 1 unit) + (21 + 1 unit)
Difference in age between Mike and KH ----- 21 + 1 unit

Difference in age between two people remains the same.

30 – 2 units ----- 21 + 1 unit

3 units ----- 9
1 unit ----- 3

When Mike’s age was 30, Bala was 3 years old.

62 – 30 ------ 32

Therefore , Bala ----- 3 + 32 ------ 35 years old.

Best wishes

[kkme]

Thks tianzu!

tianzhu:

kkme:

1) Tom's present age is thrice Sally's age some years ago. Conversely, Sally's present age is Tom's age the same number of years ago. Given that the sum of their present ages is 30 years, how old is Tom now?

Hi

For Q1, you may use MD or Units and Parts.

Due to time constraints, I’ll skip the MD this time.

At present
Tom ----- 3 units
Sally ----- 1 part

Some years ago
Tom ----- 1 part
Sally ----- 1 unit

The difference in age between two people remains the same.

3 units – 1 part ----- 1 part – 1 unit

4 units ----- 2 parts
2 units ----- 1 part

The sum of their present ages is 30 years.

3 units + 2 units ----- 30
5 units ----- 30
1 unit ----- 6

Tom ----- 3 units ----- 18

Best wishes

[mathnoobs]

tianzhu:

kkme:

1) Tom's present age is thrice Sally's age some years ago. Conversely, Sally's present age is Tom's age the same number of years ago. Given that the sum of their present ages is 30 years, how old is Tom now?

Hi

For Q1, you may use MD or Units and Parts.

Due to time constraints, I’ll skip the MD this time.

At present
Tom ----- 3 units
Sally ----- 1 part

Some years ago
Tom ----- 1 part
Sally ----- 1 unit

The difference in age between two people remains the same.

3 units – 1 part ----- 1 part – 1 unit

4 units ----- 2 parts
2 units ----- 1 part

The sum of their present ages is 30 years.

3 units + 2 units ----- 30
5 units ----- 30
1 unit ----- 6

Tom ----- 3 units ----- 18

Best wishes

Hi Tianzhu
how does one know when to use units and parts ? Here the units are assigned to Sally, and parts to Tom. However, since Tom has age in both present time and some other time, how does one tell whether to assign parts/units based on time or individual ? What is the rational for the assignment ?

[tianzhu]

mathnoobs:

Hi Tianzhu
how does one know when to use units and parts ? Here the units are assigned to Sally, and parts to Tom. However, since Tom has age in both present time and some other time, how does one tell whether to assign parts/units based on time or individual ? What is the rational for the assignment ?

Hi

I am sorry. I can’t answer in detail at this moment.

If you have a copy of Challenging Maths Made Easy, try to do some reading first.

I’ll try to answer your question at a later time.

Best wishes

[usaik]

I need help on the following question : -


K had 64 coins in his coin box. There are 20-cent and 50-cent coins. Total value of all coins was $18.50. How many 20-cent coins and how many 50-cents coins were there?

Appreciate very much your kind help.

[tianzhu]

usaik:

I need help on the following question : -


K had 64 coins in his coin box. There are 20-cent and 50-cent coins. Total value of all coins was $18.50. How many 20-cent coins and how many 50-cents coins were there?

Appreciate very much your kind help.

Hi

You may use “Make a Supposition”

Assume all were 20 cents coins, we’ve 64*20 ------ 1280

Difference between 50cents and 20 cents ------ 30 cents

1850 – 1280 ----- 570
759/30 ------ 19

19 50 cents coins
45 20 cents coins

For more practice, please visit pg 25-27 of this link.

http://www.kiasuparents.com/kiasu/forum/viewtopic.php?f=67&t=25121&start=240

Best wishes

[MathIzzzFun]

usaik:

I need help on the following question : -


K had 64 coins in his coin box. There are 20-cent and 50-cent coins. Total value of all coins was $18.50. How many 20-cent coins and how many 50-cents coins were there?

Appreciate very much your kind help.

Hi

using area model ...

http://i41.tinypic.com/vdnx4y.jpg\">

cheers.

[usaik]

Hi Tianzhu & MathIzzzFun,


Thank you so much for your solution!

Regards.

[mathnoobs]

tianzhu:

mathnoobs:

Hi Tianzhu
how does one know when to use units and parts ? Here the units are assigned to Sally, and parts to Tom. However, since Tom has age in both present time and some other time, how does one tell whether to assign parts/units based on time or individual ? What is the rational for the assignment ?

Hi

I am sorry. I can’t answer in detail at this moment.

If you have a copy of Challenging Maths Made Easy, try to do some reading first.

I’ll try to answer your question at a later time.

Best wishes

Hi Tianzhu
no problem. Thanks for responding first.
I do have the Challenging Maths Made Easy, and here is why I have the confusion.
It is only in Chapter 7, External Transfer with Changed Quantities Concept, that I see both Units and Parts being used.
In the others, ie. Unchanged Total, Unchanged Difference, Unchanged Quantity, only Units are used for the typical Before and After cases. In the unchanged scenarios, the 2 ratios are linked by normalizing them to a common denominator. But before the normalization, I would have thought the After scenario would be labelled as Parts rather than Units.

In the age problem, I see 2 scenarios, the Present age scenario and the some other time scenario. I would have thought that based on the Chapter 7 concept, the label Units, Parts would be assigned based on scenarios (ie. Before, After) in thise case (Present Age, Some Other Time)

[mathnoobs]

after some thinking, I think I see the logic.

The labels Units/Parts are assigned based on the comparison made by the question.
Since the question compared Sally’s age at present time to Tom’s age at some other time, then that comparison should be assigned the same label (ie. either Units or Parts) The second comparision would be assigned another label.

However, I still have this question: in Challenging Maths Made Easy, the chapters on Unchanged Total/Difference/Quantity all used only 1 label for both the Before and After comparision. Should not they have used different labels for the Before and After cases ? I can understand that after normalization (lowest common multiple), all ratios are assigned a single label. But before they are normalized, shouldn’t they have different labels ?