Q&A - PSLE Math

[lizawa]

pixiedust:

Would greatly appreciate some help with the following question. I need solution withOUT using Unit Transfer please.

Thank you :

Larry had some red and blue marbles in the ratio 2:5. After he bought another 30 red marbles and 15 blue marbles, the ratio became 4:7. How many marbles did Larry have at first ? Answer : 175 marbles

Red : Blue
= 2 : 5
(+30 red and + 15 blue)
= 2u + 30 : 5u + 15
= 4 :7

7(2u +30) = 4 (5u+15)
14u +210 = 20u +60
6u = 150
1u = 25
AT first -> 7u = 7 x 25 = 175.

[Brenda10]

pixiedust:

Would greatly appreciate some help with the following question. I need solution withOUT using Unit Transfer please.

Thank you :

Larry had some red and blue marbles in the ratio 2:5. After he bought another 30 red marbles and 15 blue marbles, the ratio became 4:7. How many marbles did Larry have at first ? Answer : 175 marbles

Hi pixiedust

Sorry, deleted as wrong post.

[pixiedust]

lizawa, thanks...but how to explain to P5/6 this step :

\"= 2u + 30 : 5u + 15
= 4 :7
7(2u +30) = 4 (5u+15)..\"

Brenda, no problem. If your dd has any other way to solve, do update too. thanks in advance.

[wkong]

MathIzzzFun:

Hi

http://i52.tinypic.com/161aijn.jpg\">

cheers.

Thanks again MathIzzzFun

[tianzhu]

pixiedust:

Would greatly appreciate some help with the following question. I need solution withOUT using Unit Transfer please.

Thank you :

Larry had some red and blue marbles in the ratio 2:5. After he bought another 30 red marbles and 15 blue marbles, the ratio became 4:7. How many marbles did Larry have at first ? Answer : 175 marbles

Hi pixiedust

In this question, different numbers of marbles are added to the initial quantities; hence the total number of marbles (red and blue) in the end is not the same as in the beginning.

You may use MD or Units and Parts (UP) to solve it.

Due to time constraints, I shall skip drawing the MD but would describe the steps as in MD.

Draw 2 boxes to show red marbles and 5 similar boxes to show blue marbles. Further divide these boxes into 4 boxes and 10 boxes using equivalent ratios.

Draw another box to show 30 red marbles and add to the 4 boxes(red).At the same time, add another box showing 15 marbles to the 10 boxes(blue).
So we have 4 red boxes plus 30 red marbles and 10 blue boxes plus 15 blue marbles.

In the end we have 1 red box plus 7.5(30/4) to show 1 part
We have 4 red boxes plus 4 boxes of 7.5 and 7 blue boxes plus 7 boxes of 7.5 in the end. This shows the final ratio of 4:7.

Comparing,we have 10 units + 15 equal to 7 units + 7*(7.5), giving 1 unit equals to 12.5.

Larry has 14 units or (14*12.5) which is 175.

If you have a copy of this book, Challenging Maths Made Easy, you may also refer to the solution using UP.It’s quite similar to lizawa’s solution. Instead of cross multiplication and expansion of brackets, it uses equalisation of parts as a comparison.You may refer to a solution of a similar type of question using UP in one of the earlier posts.

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Best wishes

[pixiedust]

tianzhu, thank you very much !

I am struggling at the last step of the MD, I will draw it out again to get your point.

I do have the book and explored the UP method. However, I cannot find an effective way to explain (1) why is it necessary to make the parts the same - in your diagram ie. 16 parts or 28parts in my problem sum (2) for questions with -ve quantity, the part must be moved to the other identity. I am afraid if I cannot explain the rational behind these 2 steps, ds will just memorise the steps and unable to apply UP method effective. Would appreciate any suggestions.

[MathIzzzFun]

pixiedust:

Would greatly appreciate some help with the following question. I need solution withOUT using Unit Transfer please.

Thank you :

Larry had some red and blue marbles in the ratio 2:5. After he bought another 30 red marbles and 15 blue marbles, the ratio became 4:7. How many marbles did Larry have at first ? Answer : 175 marbles

Hi pixiedust,

http://i54.tinypic.com/oi9isi.jpg\">

cheers.

[wkong]

Need help for this tricky maths question:


Joy has 4 times as many blue beads as yellows beads. For every 5 blue beads she used to make a necklace, she used 3 yellow beads. She completed the necklace after she used up all the yellow beads. If she has 35 blue beads left, how many yellow beads did she has at first?

Thanks in advance,
wkong

[MathIzzzFun]

wkong:

Need help for this tricky maths question:


Joy has 4 times as many blue beads as yellows beads. For every 5 blue beads she used to make a necklace, she used 3 yellow beads. She completed the necklace after she used up all the yellow beads. If she has 35 blue beads left, how many yellow beads did she has at first?

Thanks in advance,
wkong

Hi

http://i52.tinypic.com/j8mo01.jpg\">

cheers.

[tianzhu]

pixiedust:

tianzhu, thank you very much !

I am struggling at the last step of the MD, I will draw it out again to get your point.

Hi pixiedust

As usual, I’ve drawn more MD than necessary to show the flow of events.

Hope this helps.

Best wishes

http://farm7.static.flickr.com/6144/5949729319_2a4847247d_z.jpg\">