Q&A - PSLE Math

[Tang]

cimman:

Tang:

Hi,
You may like to check your workings.
The total number of coins before transfer and after transfer cannot be the same.
The total value / amount before transfer and after transfer should be the same in this case.

Have a nice weekend.

Cheer!

I interpreted this statement \"Some 50-cent coins were taken out and exchanged for 20-cent coins.\" to mean that there is a one to one exchange of 50 cent coins for 20 cent coins.

If the total number of coins before and after the transfer cannot be the same, how is this constraint stated in the problem statement ?

Hi,

It means that for every two 50-cent coins, they will be exchanged for five 20-cent coins, i.e. one dollar for one dollar exchange.

[comfy]

Can someone pl help me solve this qn? TIA


Mary bought a total of 50 books and pens. Each book cost $7.80 and each pen cost $5.30. If she bought 50 books, she had to pay $20 more. How many books and pens did she buy at first?

[PiggyLalala]

comfy:

Can someone pl help me solve this qn? TIA


Mary bought a total of 50 books and pens. Each book cost $7.80 and each pen cost $5.30. If she bought 50 books, she had to pay $20 more. How many books and pens did she buy at first?

let me try to solve
Difference in price between a book and a pen= 7.80 - 5.30 = 2.50
Difference in total if buying 50 books = 20
Extra books bought = 20/2.5 =8
Therefore she bought 50 - 8 = 42 books and 8 pens

[comfy]

I understand the step till extra books bought 20/2.5=8

But can explain the next step? why need to 50-8?

[bobsg01]

Need help with this question.


Thanks in advance.

http://i54.tinypic.com/2upvds0.jpg\">

[tianzhu]

comfy:

Mary bought a total of 50 books and pens. Each book cost $7.80 and each pen cost $5.30. If she bought 50 books, she had to pay $20 more. How many books and pens did she buy at first?

Hi

Another way to look at it is to rewrite the question in another way.

50*7.8 ------390
390-20 -----370

Now rewrite the question as follows:

Mary bought a total of 50 books and pens. Each book cost $7.80 and each pen cost $5.30. She spent $370. How many books and pens did she buy at first?

Does this look familiar? It’s a common “Make a Supposition” or “Counting heads and legs” type of question.

The answer is 42 books and 8 pens.

Best wishes

[MathIzzzFun]

bobsg01:

Need help with this question.


Thanks in advance.

http://i54.tinypic.com/2upvds0.jpg\">

Hi

this was discussed in an earlier post.. http://www.flickr.com/photos/62167097@N02/5842021695/in/photostream

cheers.

[PiggyLalala]

comfy:

I understand the step till extra books bought 20/2.5=8

But can explain the next step? why need to 50-8?

Let me try again.
8 extra books here mean if Mary buys 50 books, then it will be 8 more than the number of books she bought at first.

Hence the number of books that mary bought at first = 50 - 8 = 42.

Sorry i am not very good in explaining how i arrive at my answer. Hope you can understand. Maybe others can help. You can also look at Tianzhu's solution.

[PiggyLalala]

tianzhu:

comfy:

Mary bought a total of 50 books and pens. Each book cost $7.80 and each pen cost $5.30. If she bought 50 books, she had to pay $20 more. How many books and pens did she buy at first?

Hi

Another way to look at it is to rewrite the question in another way.

50*7.8 ------390
390-20 -----370

Now rewrite the question as follows:

Mary bought a total of 50 books and pens. Each book cost $7.80 and each pen cost $5.30. She spent $370. How many books and pens did she buy at first?

Does this look familiar? It’s a common “Make a Supposition” or “Counting heads and legs” type of question.

The answer is 42 books and 8 pens.

Best wishes

Thank you for showing us another way doing this question.
I love your way of transforming a difficult question to one that students are familiar with. I feel that this way of teaching is especially good for the weaker students.

Good luck to all your students in their coming PSLE too. :rahrah:

[tianzhu]

PiggyLalala:

Thank you for showing us another way doing this question.

I love your way of transforming a difficult question to one that students are familiar with. I feel that this way of teaching is especially good for the weaker students.

Good luck to all your students in their coming PSLE too. :rahrah:

Hi

You're welcome.

I think some students may be caught off guard by this question.
It is actually a familiar question with a slight tweak.

The assumption, “Assume all were books” in this particular question is written as a statement in the question.

Students who understand the concepts should be able to see that the additional number of books is equal to the number of pens. The $20 is due to the book costing $2.50 more than the pen.

My students?

I've no students.

I am not a teacher, I am a simple dad of a post PSLE kid.

Best wishes