Q&A - P3 Math

[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.

[cimman]

tianzhu:

cwc:

Hi Tianzhu,
Very curious, what do you use to produce all these fantastic diagrams/pics to explain?
Impressive :salute: How much time do you need to typically come up with one?

Hi cwc

Good Morning.

I am happy you liked my “masak masak” way of doing maths.

Just like any solutions in this forum, you’re correct to say that efforts are put in to post them.

I am a simple daddy, so I am not very conversant with IT. At times I need to play around with the software which I used in my solutions, which is PowerPoint in this case.

The usual process starts with coming up with some thoughts on the mathematical solutions to the questions. The thoughts are then translated into usually pictorial presentation which I hope will help students see that Maths is beautiful.

Best wishes

you should check out Microsoft Visio. It's a lot easier drawing the stuff you draw in Visio then in Powerpoint, and it's a lot faster as well.

[tianzhu]

Thein:

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?

Hi

I think it’s less efficient to use MM(Multiples method) to solve this question.

You may apply the concept of Gap and Difference.

Gap ------- 1 + 3 ------ 4
Difference in number of erasers -------- 8 - 4 ------- 4

Cost of 1 eraser ------- 4/4 ------- 1
Amount of money -------- 4 + 1 ------- 5

Best wishes

[tianzhu]

Champion:

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

Hi Champion

You're welcome.

Best wishes