Q&A - PSLE Math

[MathIzzzFun]

Oracle:

Thanks a lot MathIzzzFun and SpeedMaths (still up at 1:54am!!! Phew ...)

Will show them to my daughter tonight and see if she is able to relate.

The other two questions:


Q5. Mr Tan took 6 hours to travel from Town A to Town B. Mr Lim took 4 hours to travel from Town B to Town A. If both of them started travelling at 10 am, at what time did they pass each other?

Thank you.

Time taken to travel the distance between A and B,
Mr Tan's time : Mr Lim's time = 6 : 4 = 3 : 2

Mr Tan's speed : Mr Lim's speed --> 2 : 3

When Mr Tan and Mr Lim passed each other,
Mr Tan has travelled 2 units from Town A, and
Mr Lim has travelled 3 units from Town B

Total distance between Town A and B = 2 units + 3 units = 5 units

Time taken for Mr Tan to travel 2 units = 2/5 x 6 h = 2.4h = 2h 24 min

10 am + 2h 24 min ---> 12.24pm

Mr Tan and Mr Lim passed each other at 12.24pm

cheers.

[Mary Joy]

Pls help me with this circle question…


two wheels with centres A and B are 641 cm apart.The radius of the small wheel is 7 cm.The ratio of the radius of the small wheel to the ratio of the big wheel is 1:3.Both the wheels are rolled in the inward direction until the centres are 113 cm apart. The two wheels need to make the same no of revolutions.

How many revolutions does each wheel make assuming that each wheel must make a complete revolution? (take pi:3.14)

[iCreative Math]

Radius of small wheel = 7 cm

Radius of big wheel = 21 cm (given ratio of 1:3)

In one complete revolution each, the small and big wheels would have travelled a combined distance of 2 X 3.14 X 28 = 175.84

For the centre to be 113 cm apart, the total distance travelled would have to be 641 - 113 = 528 cm.

The wheel would have each make 528 / 175.84 = 3 complete revolutions each.


Regards.
iCreative Math

[Easy-going]

can anyone help me with this problem sum please?

The ratio of the number of red markers to the number of blue markers in Box ia 3:1. The ratio of the number of red markers to the number of blue markers i Box B is 2:3. There are twice as many markers in Box than Box B.
A) What is the ratio of the number of red markers in Box A to the number of red markers in Box B?

B) 60 red markers are added into Box B and the ratio of the number of red markers to the number of blue markers in Box B becomes 4:1. How many blue markers are there in Box B?

Thanks in Advance

[PapayaDad]

B)

Red = 2u ; Blue = 3u
At first 2u:3u
Change +60 : 0
In the end 4p:1p
So
2u+60.....4p
3u.....1p
Make the part the same
2u+60.....4p
12u....4p
12u = 2u+60
u=6
Blue = 3u = 18

See http://www.kiasuparents.com/kiasu/content/math-questions-involving-two-changed-quantities

[PapayaDad]

can anyone help me with this problem sum please?

The ratio of the number of red markers to the number of blue markers in Box ia 3:1. The ratio of the number of red markers to the number of blue markers i Box B is 2:3. There are twice as many markers in Box than Box B.
A) What is the ratio of the number of red markers in Box A to the number of red markers in Box B?

Box A:
r=3u
b=1u
Total = 4u

Box B:
r=2p
b=3p
Total = 5p

There are twice as many markers in Box A than Box B. So
4u = 5p x 2 = 10p
u=2.5p

In Box A:
r=3u so r=3 x 2.5p = 7.5p

In Box B:
r=2p

Ratio is 7.5 to 2 or 15:4

[Oracle]

Thanks oldschool for the lead. The materials are informative indeed.


MathIzzzFun, re Ann and Ben question, may I know how you derived 1 unit ~ 24km?

[MathIzzzFun]

Oracle:

Thanks oldschool for the lead. The materials are informative indeed.


MathIzzzFun, re Ann and Ben question, may I know how you derived 1 unit ~ 24km?

hope this is legible...

http://i49.tinypic.com/148m4aw.png\">

cheers

[Musicstar]

Hi,


Can someone help to check whether this question any problem? I have the answer but still do not understand . Why the ratio for 7 years ago change to 4 : 2?



Question:
Seven years ago, the ratio of the Samuel’s age to George’s age was 2: 1. The ratio of their ages is 5 : 3 this year. What will be the ratio of their ages in seven years time?

Answer:
7 years ago
S : G
2 : 1
4 : 2

Now
S : G
5 : 3

diff 5 -4 =1
1u = 7
5u = 5 x 7
= 35
3u = 3 x 7
=21
Samuel is 35 yrs old & George is 21 yrs old.

7 years time
s : G
35 + 7 = 42 21 + 7 = 28
3 : 2
The ratio of their ages wil be 3: 2

[PapayaDad]

7 years ago 2u:1u

Now (change) +7:+7
End 5p:3p

2u+7…5p
1u+7…3p

Make the part the same (cross multiply)

6u+21…15p
5u+35…15p

6u+21 = 5u+35
u=14

7 years ago 2u:1u
7 years ago 28:14
Now 35:21
7 years later 42:28
7 years later 3:2