Q&A - P3 Math

[MathIzzzFun]

AgonyMum:

Are there any Math teachers/ experts who can help explain these 4 questions to me? I have asked another resident expert and I am still not \"catching\" it.

Please help! Thanks very much in advance! Please provide detailed explanation as my child and I are both new to this topic of \"Working Backwards\".


3. A owned a gigantic bookshelf stuffed with books.It had 3 sections,the bottom section, the middle section and the top section. There were a total of 192 books on the shelf. From the top section, A transferred to the middle section the number of books the middle section originally had. Then, from the middle section,he transferred to the bottom section the number of books the bottom section had originally. Lastly, from the bottom section, he transferred to the top section the number of books that the top section contained after the first transfer. Now, the number of books in all 3 sections are equal. How many books were there in the top, middle and bottom section respectively at the very beginning?

Hi

In the end, there were equal number of books on each shelf -> 192 / 3 = 64

Before transfer of books from bottom shelf, books on top shelf = 64 / 2 = 32 (* the final transfer doubles the books on top shelf, so to restore, ½ the number)

Before transfer of books to top shelf, books on bottom shelf = 64 + 32 = 96 (*add back the number that was transferred to top shelf*)

Before transfer of books from middle shelf, books on bottom shelf = 96/ 2 = 48 (*number of books on bottom shelf was doubled with transfer from middle shelf, so to restore, ½ the number)

Before transfer of books to bottom shelf, books on middle shelf = 64 + 48 = 112 (*add back the number that was transferred to bottom shelf*)

Before transfer of books from top shelf, books on middle shelf = 112/2 = 56 (*number of books on middle shelf was doubled with transfer from top shelf, so to restore, ½ the number)


Before transfer of books to middle shelf, books on top shelf = 32 + 56 = 88 (*add back the number of books that were transferred to middle shelf)

At first,
Top shelf -> 88 books
Middle shelf -> 56 books
Bottom shelf -> 48 books

in pictorial form -> http://tinyurl.com/mathizzzfun-20120426workback01


cheers.

[MathIzzzFun]

AgonyMum:

Oh, after applying you method for Qn 2 , I can solve Qn 1 too Don't know abt Qn 3 and Qn 4 though.....

YEAH!!!
MathIzzzFun,you really do make Maths fun.You could not have chosen a better nic.
THANKS!

u r welcome

cheers.

[AgonyMum]

Hi MathIzzzFun,

Am still trying to figure out Qn 3 and 4 but the good news is my child has understood Qn 1 and 2....All thanks to you!

[AgonyMum]

Hi,

What is the fastest and most efficient of solving questions like this one:

pen+pencil+eraser=630g
pen+ruler +pencil=730g
eraser +pencil+ruler=330g
pen+eraser+ruler=800g
What is the value of pen+pencil+eraser+ruler?
Is simult. equation with substitution the fastest way?
I tried to solve it and although it can be done, it is pretty tedious.

TQ in advance.

[MathIzzzFun]

AgonyMum:

Hi,

What is the fastest and most efficient of solving questions like this one:

pen+pencil+eraser=630g
pen+ruler +pencil=730g
eraser +pencil+ruler=330g
pen+eraser+ruler=800g
What is the value of pen+pencil+eraser+ruler?
Is simult. equation with substitution the fastest way?
I tried to solve it and although it can be done, it is pretty tedious.

TQ in advance.

Hi

add all to get 3 sets of (pencil, eraser, ruler,pen) and divide by 3.

cheers.

[AgonyMum]

Oh yah! Got it.You really make it seems so easy!

Thanks a million,MathIzzzFun

[BigDevil]

Hi AgonyMum,


Please post your future Math questions in the http://www.kiasuparents.com/kiasu/forum/viewtopic.php?f=66&t=149 thread.

This thread will soon be merged with the Q&A thread.

Thank you.

[AgonyMum]

Hi BigDevil, apologies for posting in the wrong thread.Will post at P3 Math in future.

[MathIzzzFun]

AgonyMum:

Hi MathIzzzFun,

Am still trying to figure out Qn 3 and 4 but the good news is my child has understood Qn 1 and 2....All thanks to you!

Hi

Q3 ... solution in pictorial

http://tinyurl.com/mathizzzfun-20120426workback01

cheers.

[yuanyuan mama]

Hi,


Below is the question given by my child's teacher in a Math exercise:

In a class of 42 pupils, each boy received 3 stickers and each girl received 4 stickers from teacher. Altogether, the teacher gave out 154 stickers. How many girls were there in this class?

I know how to solve by guess and check but not the solution below (given by teacher).

42 (tot pupils) X 3 (stickers for each boy) = 126
154 (tot stickers given out) - 126 = 28
ANSWER: 28 girls in the class

Can someone help to explain why 42 X 3 and not X 4 (sticker for each gal)? And why 28 is the number of gals, not stickers received by gals? Does this formula always work for guess and check question?

Hope someone may enlighten me. Thanks.