Tutor MathsGuru: Ask me for your burning Maths questions!

[OngMum]

Thank you, Tang for the link on the correct qns phrasing.

Thank you, Mathsguru for the problems solving.

Have a good day!

Cheers!

[YLH88]

Hi Mathsguru,


Thank you for the solution!

Regards

[firebird]

Dear Coffeecat and Dharma


Thank you for the answer.

1) Coffeecat, for the block question, I got the answer 180 = 90 x 2 x 1
Your working is rather too tough for me to follow. Neverheless, I will try again and again until this weekend - 29/3/10. If sill lost, I will ask for help.

2) Dharma, I was able to follow your working for the time quesion on a second attempt.

Many thanks.

Regards
firebird

[Dharma]

firebird:

Dear Coffeecat and Dharma


Thank you for the answer.

1) Coffeecat, for the block question, I got the answer 180 = 90 x 2 x 1
Your working is rather too tough for me to follow. Neverheless, I will try again and again until this weekend - 29/3/10. If sill lost, I will ask for help.

2) Dharma, I was able to follow your working for the time quesion on a second attempt.

Many thanks.


Regards
firebird

Hi firebird,

Coffeecat has given you a detailed explanation considering the different cases. I’ll just try to simply his explanation and hopefully you can follow.

90 divide by 3 = 30
50 divide by 2 = 25
70 divide by 4 = 17R2 (You are left with remainder of 2cm uncut)

If u cut the 50cm x 70cm x 90cm in such a way where little cuboid of side 3cm is along the 90cm of the block, 2cm side of little cuboid along the 50cm of the block and 4cm side of the little cuboid is along the 70cm of the block.

Total no. of small cuboids cut-out = 30 x 25 x 17 = 12,750
Total volume of cuboids cut-out = 12,750 x (2 x 3 x 4)cm3 = 306,000cm3

Volume of block left = (50 x 70 x 90)cm3 – 306,000cm3 = 9000cm3

OR

Volume left = 90cm x 50cm x 2cm = 9000cm3

You will realize that after cutting off the little cuboids, you are left with a block of 90cm x 50cm x 2cm. We cut this block again into smaller cuboids of 2cm x 3cm x 4cm.

90 divide by 3 = 30
50 divide by 4 = 12R2 (You are left with remainder of 2cm uncut)

No. of small cuboids cut-out = 30 x 12 x 1 = 360
Volume of cuboids cut-out = 360 x (2 x 3 x 4) cm3 = 8640cm3

Amount/volume of wasted wood = 9000cm3 – 8640cm3 = 360cm3

OR
Volume left/wasted = 90cm x 2cm x 2cm = 360cm3

[tianzhu]

Hi firebird


At first glance, this question may look ordinary.But it may not be so.

To minimize wastage, you need to cut out the maximum number of little cuboids.

Hope this helps.

Best wishes


http://farm5.static.flickr.com/4071/4458057147_95b6c1e7db_o.jpg\">

[firebird]

Dear Coffeecat, Dharma & Tianzhu


I thank you very much for the detailed explaination and working posted for the block question.

With best regards
firebird

[firebird]

Dear mathsguru


Please help me on the following P6 maths:

1) Sophie and Rachel share blue ribbons in the ratio of 5 : 2.
They want to buy some red ribbons so that the total number of ribbons they have will be tripled. In what ratio must they divide the red ribbons so that Rachel will have twice the number of ribbons she has now?

I got 17:4. But the given answer is 6:1.

2) Faizal has a rod. He needs 6 minutes to saw it into 4 pieces. How many minutes does he need to saw the rod into 28 pieces?

I got 42 minutes. But the given answer is 54 minutes

3) Customers were given a free CD for every 2 CDs they buy at a sale.
Ben spent $K and got 30CDs in all. WHat is the price of 1 CD?

I got $K/20. But the given answer is $K/10

4) There were some horses and people on a farm. There were 62 more legs than heads. There were 26 more heads than tails.

a) How many people were there?
b) How many horses were there?

I really do not know where to start for question 4 a) & b)

5) Sugar and water are mixed in the ratio 1:5 to make syrup. The volume of the syrup is 800 cm3.

a) Find the volume of sugar in the syrup.
b) Find the volume of water in the syrup.
c) How much more sugar must be added to the surup so that the ratio of the sugar to water in the syrup becomes 1:3?


a) I got 133 1/3 Cm3 But given answer 13 1/3 CM3. I hope i am right.
b) I got 666 2/3 cm3 and the given answer also 666 2/3 cm3.
c) I got 1,866 2/3 cm3 But the given answer 88 8/9 cm3


Many thanks
firebird

[Vanilla Cake]

YLH88:

[quote]Hi YLH88,

While waiting for Mathsguru's model solution, pls see whether mine using ratio is of any use or not?

(a)
D : T
8 : 11
16 : 22

D : T
19 : 19

(16+22)÷2=19

3u=18
u=6
6u=36

At first, Thomas had 36 more game cards than Doran.

(b)
D : T
19:19
38: 38

D : T
3 : 1
57:19

6u=18 [from part (a) ie 6u=36, reduce half of 36 = 18 since 19:19 has been doubled to 38:38 in part (b)]
u=3
19u = 57
Thomas lost 57 more cards to Doran.

Hi Vanilla Cake,

For (a), I got the answer using the model method. For the unit method, why did you change 8:11 to 16:22 ? Why do we multiple by 2 and not 3 or 4 etc ?

For (b), why do you change from 19:19 to 38:38 and 3:1 to 57:19 ?

Thank you![/quote]Hi YLH88,
Sorry for the late reply due to my busy schoolwork.
This is a 4-mark question is from Nanyang 2009 P6 SA1 Paper 2 Q12.

(a) You need to multiply 8:11 by 2 to get 16:22.Add 16+22=38 which is the same as 19+19=38.
(b)19:19 multiply by 2 to get 38:38 so that 38+38=76. And 3:1 multiply by 19 to get 57:19 so that 57+19=76.

[Tang]

Vanilla Cake:

CoffeeCat:

i think vanilla is hoping for mathsguru to come up with an alternative solution based on models.

Yes, CoffeeCat. You are right, I am looking for alternative solution based on models from Mathsguru besides the solution using simultaneous equations as posted by Tang.

Hi Vanilla Cake,

I believe this is the model drawing you are looking for.


http://psle2010a.blogspot.com/2010/03/percentage_24.html


Hope it is useful for Pandan Cake.

[Vanilla Cake]

firebird:

1) Sophie and Rachel share blue ribbons in the ratio of 5 : 2.They want to buy some red ribbons so that the total number of ribbons they have will be tripled. In what ratio must they divide the red ribbons so that Rachel will have twice the number of ribbons she has now?

A1

Before
S : R
5 : 2

Total->5+2=7.
To buy additional red ribbons so that the total number of ribbons will be tripled means 7x3=21.

After
S : R
17 : 4 (Rachel will have twice the number of ribbons->2x2=4, remainder will be 21-4=17 for Sophie)

For Sophie: (17-5) units = 12 units
For Rachel: (4-2) units = 2 units

12: 2 = 6:1
Ratio for them to divide the red ribbons is 6:1

firebird:

2) Faizal has a rod. He needs 6 minutes to saw it into 4 pieces. How many minutes does he need to saw the rod into 28 pieces?

A2

To get 4 pieces of the rod, Faizal needs to saw 3 times.
To get 28 pieces of the rod, Faizal needs to saw 27 times.

To saw the rod for 3 times -> 6 minutes
To saw the rod for 27 times -> 27/3x6 = 9x6 = 54 minutes

Faizal will need 54 minutes to saw the rod into 28 pieces.

firebird:

3) Customers were given a free CD for every 2 CDs they buy at a sale.Ben spent $K and got 30 CDs in all. What is the price of 1 CD?

A3

30/(2+1)x2 = 20 means 20 CDs are bought and 10 CDs were given free.
Total amount that Ben spent was $K.
Price of 1 CD = $(K/20)

firebird:

4) There were some horses and people on a farm. There were 62 more legs than heads. There were 26 more heads than tails.
a) How many people were there?
b) How many horses were there?

A4
There were 26 more people than the horses.
(Horse : 1 head + 1 tail)
(Person: 1 head only)

Head
Horse : unit
People : unit +26

Legs
Horses : 4 units
People : 2 units +52

Total heads : 2 units + 26
Total legs : 6 units + 52

6 units+52-(2 units+26) = 62 (Given that 62 more legs than heads)
6 units+52-2 units-26=62
4 units+26=62
4 units = 36
1 unit = 9

(a) Number of people would be 26+9 = 35
(b) Number of horses would be 9

firebird:

5) Sugar and water are mixed in the ratio 1:5 to make syrup. The volume of the syrup is 800 cm³

a) Find the volume of sugar in the syrup.
b) Find the volume of water in the syrup.
c) How much more sugar must be added to the surup so that the ratio of the sugar to water in the syrup becomes 1:3?

A5
S : W
1 : 5

6 units = 800 cm³
1 unit = (800/6) cm³ = 133⅓ cm³

(a) Volume of sugar in the syrup is 133⅓ cm³

(b) Volume of water in the syrup will be 5x133⅓ cm³ = 666⅔ cm³

Before
S : W
1 : 5
3 : 15

After
S : W
1 : 3
5 : 15

There is no change for the water as only sugar is added to the syrup.

3 units = 133⅓ cm³
2 units = 88 8/9 cm³

(c) Additional sugar to be added is 88 8/9 cm³

Hope this helps while waiting for Mathsguru to provide her solutions.