Q&A - P3 Math

[Thein]

Hi Tianzhu


I need a help.

First, I should thank you for I was able to understand model drawing only with your explanations. Thanks a lot.

i’ve taught my child, model drawing to solve problems like :
Mrs. Tay has a bag of sweets. If she gives each child 7 sweets, she will be left with 5 sweets. If she gives each child 9 sweets, she will need 5 more sweets.
a) How many children are there?
b) How many sweets are there?

but for this problem:

Mrs. Raju bought a sack of rice which weighed less than 50kg. If she packed the rice into bags of 5kg each, she would have 3kg of rice left. If she packed it into bags of 7kg each, she would have 4kg of rice left. How many kg of rice did Mrs. Raju buy?

i couldn’t draw the model.

i’ve to go for another heuristic approach. finding the multiples…

i don’t know how to differentiate the problems…or is there any common method that i can teach my kid…

Thank you in advance.

[corneyAmber]

Thein:

Mrs. Raju bought a sack of rice which weighed less than 50kg. If she packed the rice into bags of 5kg each, she would have 3kg of rice left. If she packed it into bags of 7kg each, she would have 4kg of rice left. How many kg of rice did Mrs. Raju buy?

Here is a possible solution:
For this question, you can apply Guess and Check method as 50kg is given as the upper limit, a table can be drawn.

7X6 = 42+4 = 46 => packed into 5, R=1
7x5 = 35+4 = 39 => packed into 5, R=4
7x4 = 28+4 = 32 => R = 2
:
:
7x2 = 14+4 = 18 => R=3

So answer is 18kg.

[Champion]

tianzhu:

Hi


For Q1, you may use a heuristics known as “Make a Supposition”

For every wrong question, he lost (3+1) or 4 marks.

Assume or pretend that Joseph answer 20 questions correctly

20 x 3 ------- 60
60 – 48 ------ 12

Number of wrong questions ------- 12/4 ------- 3
Number of correct answers --------20 – 3 ------17

http://farm6.static.flickr.com/5107/5682971233_63f45f06ba_z.jpg\">

The second question touches on “Gap and Difference”

Look at this question from another angle, if Mrs Tay gives 2 more sweets (9-7) to her children, she would need (5+5) or 10 more sweets.

10/2 ------ 5
There are 5 children.

Number of sweets ------ (5*7) + 5 ------ 47 or (5*9) – 5 ------- 40

Best wishes

http://farm6.static.flickr.com/5063/5683393848_c80b1a22d7_z.jpg\">

Hi Tianzhu,
Thank you so much for your clear explanation; got it lioa! :celebrate:

[teh_oh]

ksi:

Thein:

Mrs. Raju bought a sack of rice which weighed less than 50kg. If she packed the rice into bags of 5kg each, she would have 3kg of rice left. If she packed it into bags of 7kg each, she would have 4kg of rice left. How many kg of rice did Mrs. Raju buy?

Here is a possible solution:
For this question, you can apply Guess and Check method as 50kg is given as the upper limit, a table can be drawn.

7X6 = 42+4 = 46 => packed into 5, R=1
7x5 = 35+4 = 39 => packed into 5, R=4
7x4 = 28+4 = 32 => R = 2
:
:
7x2 = 14+4 = 18 => R=3

So answer is 18kg.

5kg bags--> 10,15,20,25,30,35,40,45
Above with 3kg more--> 13,18,23,28,33,38,43,48
7kg bags--> 7,14,21,28,35,42
Above with 4kg more--> 11,18,25,32,39,46

She bought 18kg.

---
I learned this tabulation method from one of the helpful forummers; to similar question posted previously.

[Thein]

Thank you ksi.


my question is that this method be applied to the other problems too…

i understand that if there is shortage we have to subtract…but my kid…

i’m looking for a method which i can apply for both the problems…or i have to teach to differentiate the problems…

Thank you once again…and expecting much more help from you…

Thank you teh_oh…
i’ll try to explain this method to my kid…

[Thein]

hi teh_oh


i taught this method earlier itself...

but she got this problem as her homework in onsponge workbook:

Tina had some money. If she buys4 erasers she has $1 left. If she buys 8 erasers with the same amount of money she needs $3 more. How much money does Tina have and what is the cost of one eraser?

multiples of 4: 4 8 12 16 20 24
+1 : 5 9 13 17 21 25
multipes of 8: 8 16 24 32 40
-3: 5 13 21 29 37

my kid confused with the 3 matches.

Teacher also asked to erase this and to draw model.
Is it like i've to take the first match....
Thank you in advance.

[tianzhu]

Thein:

Hi Tianzhu


I need a help.

First, I should thank you for I was able to understand model drawing only with your explanations. Thanks a lot.

but for this problem:

Mrs. Raju bought a sack of rice which weighed less than 50kg. If she packed the rice into bags of 5kg each, she would have 3kg of rice left. If she packed it into bags of 7kg each, she would have 4kg of rice left. How many kg of rice did Mrs. Raju buy?

i couldn't draw the model.

i've to go for another heuristic approach. finding the multiples....

i don't know how to differentiate the problems...or is there any common method that i can teach my kid...

Thank you in advance.

Hi Thein

You're welcome.

A similar question to the Raju's question has been discussed in one of the earlier posts in the PSLE Maths thread.Here's the solution.

http://farm6.static.flickr.com/5185/5590692977_b2c5469dc2_z.jpg\">

I'll try to consolidate my thoughts and attempt to answer your query tomorrow.

Best wishes

[MathIzzFun]

Thein:

hi teh_oh


i taught this method earlier itself...

but she got this problem as her homework in onsponge workbook:

Tina had some money. If she buys4 erasers she has $1 left. If she buys 8 erasers with the same amount of money she needs $3 more. How much money does Tina has and what is the cost of one eraser?

multiples of 4: 4 8 12 16 20 24
+1 : 5 9 13 17 21 25
multipes of 8: 8 16 24 32 40
-3: 5 13 21 29 37

my kid confused with the 3 matches.

Teacher also asked to erase this and to draw model.
Is it like i've to take the first match....
Thank you in advance.

Hi Thein,

When listing method is used, one has to be clear with the rationale. For eg.
multiples of 4: 4(1), 8(2), 12(3), 16(4),20(5), 24(6)
+1 : 5 9 13 17 21 25
multipes of 8: 8(1), 16(2), 24(3), 32(4), 40(5)
-3: 5 13 21 29 37

The number within bracket is the price per eraser. Although there are other matches - 13, 21 etc these actually represent different price per eraser eg. for the matching 13, looking at multiples of 4, the price per eraser is $3, whereas it is $2 per eraser looking at the multiples of 8. So the only match here is 5 which gives $1 per eraser for both multiple of 4 & 8.

Hope I didnt confuse you

cheers.

[teh_oh]

Tianzhu and MathIzzFun,

:thankyou: for clarifying

[cimman]

this is the LCM (lowest common multiple) method. There will always be a number of matching common multiples. Always pick the lowest one, unless the question qualifies the range, ie. 50 to 60.