Q&A - P5 Math

[MathIzzzFun]

kkme:

Hi, need help for these two qns.


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?

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.

Thanks in advance and Happy CNY!

Hi

MD for Q2 which is same as this question that was discussed some time ago - http://www.flickr.com/photos/62167097@N02/5679985631/in/photostream

cheers.

[mathnoobs]

I’ve a question taken from the book by Ammiel Wan, Challenging Maths Problems Made Easy. This is from the chapter on Gap and Difference, Q7.


If I buy 3 popcorn combos and 4 nacho combos, I will have $4.50 left. If I buy 4 popcorn combos and 3 nacho combos, I will have $3.50 left. Given that a nacho combo costs $6, find the amount of money I have.

Difference between popcorn combos = 1 popcorn combo
Difference between nacho combos = 1 nacho combo
Difference between costs = $1

Here is the part I don’t understand, the solution says:
1 popcorn combo - 1 nacho combo = $1 (popcorn costs more than nacho)

why not,
1 nacho combo - 1 popcorn combo = $1 ? (nacho costs more than popcorn)

how do I know which costs more, popcorn combo or nacho combo ?

[MathIzzzFun]

hi


drawing a model will help you see that popcorn cost more than the nacho ..

http://i41.tinypic.com/14bjncz.jpg\">

cheers.

[happyheart]

Urgent! Mental block.

How to explain this question using algebra substitution method? e.g 5p=4e

Q: Two identical pens cost as much as three similar erasers. Sally bought 5 such pens and 4 such erasers at $9.20.
What is the cost of 1 pen?

Thanks!

[tianzhu]

happyheart:

Urgent! Mental block.

How to explain this question using algebra substitution method? e.g 5p=4e

Q: Two identical pens cost as much as three similar erasers. Sally bought 5 such pens and 4 such erasers at $9.20.
What is the cost of 1 pen?

Thanks!

Hi

You may use “Number*Value”

Use a common multiple of 6 (2 and 3)

2 pens ----- 6 units
1 pen ------ 3 units

3 erasers ------ 6 units
1 eraser ------- 2 units

Sally bought 5 such pens and 4 such erasers at $9.20.

(5*3) + (4*2) ------ 920

23 units ------ 920
1 unit ------ 40

1 pen ------ 3 units ------- 120 cents or $1.20

Best wishes

[happyheart]

Appreciate your prompt reply, Tianzhu.


Thank you!

[mathnoobs]

Hi Mathizzfun

Thank you for taking the trouble to draw out a diagram.
I'm afraid I still don't get it.
I've drawn a model here that says Nacho combos costs more than Popcorn combos.
http://i42.tinypic.com/2laa1hj.png\">

can you tell me what is wrong with the diagram ?

Is there a way to tell which is bigger without resorting to diagram ? My child's motor skill is not very good, and has difficulty in getting the relevant boxes to be of the same size.

[MathIzzzFun]

mathnoobs:

Hi Mathizzfun

Thank you for taking the trouble to draw out a diagram.
I'm afraid I still don't get it.
I've drawn a model here that says Nacho combos costs more than Popcorn combos.
http://i42.tinypic.com/2laa1hj.png\">

can you tell me what is wrong with the diagram ?

Is there a way to tell which is bigger without resorting to diagram ? My child's motor skill is not very good, and has difficulty in getting the relevant boxes to be of the same size.

Hi

The model that you have drawn are of different length which means that the total amount is different.

Note that the total amount of money must be the same ie
total amount of money = (money spent on 4 nachos+3 popcorns) +(money left = 4.50) = (money spent on 3 nachos + 4 popcorns) + (money left = 4.50)

In fact, by noting that there is more money left if I buy 4 nachos and 3 popcorns than if I buy 3 nachos and 4 popcorns means that 3 nachos + 4 popcorns cost more than 4 nacho + 3 popcorn.

http://i40.tinypic.com/n2d1md.jpg\">

cheers.

[mathnoobs]

MathIzzzFun:

mathnoobs:

Hi Mathizzfun

Thank you for taking the trouble to draw out a diagram.
I'm afraid I still don't get it.
I've drawn a model here that says Nacho combos costs more than Popcorn combos.
http://i42.tinypic.com/2laa1hj.png\">

can you tell me what is wrong with the diagram ?

Is there a way to tell which is bigger without resorting to diagram ? My child's motor skill is not very good, and has difficulty in getting the relevant boxes to be of the same size.

Hi

The model that you have drawn are of different length which means that the total amount is different.

Note that the total amount of money must be the same ie
total amount of money = (money spent on 4 nachos+3 popcorns) +(money left = 4.50) = (money spent on 3 nachos + 4 popcorns) + (money left = 4.50)

In fact, by noting that there is more money left if I buy 4 nachos and 3 popcorns than if I buy 3 nachos and 4 popcorns means that 3 nachos + 4 popcorns cost more than 4 nacho + 3 popcorn.

http://i40.tinypic.com/n2d1md.jpg\">

cheers.

Hi MathIzzzFun
thank you very much for drawing the diagrams. It is clearer now to see the relationships. Appreciate your time in offering your help. Thank you very again.

[kkme]

Hi MathIzzzFun,


Thanks for your reply.

OMG, I still dont get it.

MathIzzzFun:

kkme:

Hi, need help for these two qns.

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?

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.

Thanks in advance and Happy CNY!

Hi

MD for Q2 which is same as this question that was discussed some time ago - http://www.flickr.com/photos/62167097@N02/5679985631/in/photostream

cheers.