Q&A - PSLE Math

[small]

andante:

I have encountered the following questions about speed :-


Two people, A and B, were running along a 400-metre oval running track. Thestarted off at the same time, from the same point and ran in the same direction. Person A ran at a speed of 280m/min while person B ran at a speed of 240m/min. How many minutes did Person A take to catch up with Person B?

I don't really understand the solution provided.

It is mentioned that
For person A to catch up with Person B, he must run one(???) more round than Person B,, that is to run an extra 400m. Since Person A covered 280 -240 = 40m more than Person B in each minute, the time he took to catch up with Person B was
400 / (280 - 240) = 10 min

Can someone help to explain ?

Thanks in advance.

Hi,

HTH..

After one min run, Person A is 40m ahead of Person B, so the distance between them is 360m.
After two min run, Person A is 80m ahead of Person B and now the distance between them is 320m.

For the remaining 320m, Person A will take another 8 min to catch up with Person B.

320 / 40 = 8

So in total Person A took 10 min to catch up with Person B.

[JadeDry]

Hello:


I would appreciate some help with this problem:

Factorize "-ab-9abc" completely.

I am stuck between two answers:

ab(-1-9c)

Or

-ab(1-9c)

I would appreciate it if someone could point out the correct one.

Thanks in advance.

[small]

Hi,


The answer is -ab(1+9c) OR ab(-1-9c).

I personally preferred the first answer.

[chrisu]

After weeks and weeks of waiting, the teacher finally confirmed that $31 is the correct answer and not $14.90. Thanks to all for helping out in this question.

tianzhu:

chrisu:

Will know by today and will revert back here once I get the working from teacher. I'm anxious to know too.

Hi

First, let’s see the teacher explanation first.

We cannot read the intention of the question setter, what we can rely on is our understanding on the concepts of percentages.

Tony had 15% more 50c coins than 20c coins. If there were 6 more 50c coins than 20c coins, how much money did Tony have?

Based on the question, we are given the understanding that the number of 50 cents coins is 15% more than the number of 20 cents coins.

Next compare your question with this common question in P6 percentage.

Jian Zhong has $50.Ming Ming has 20% more money than Jian Zhong.How much money does Ming Ming have?

How would one interpret the second question?

Best wishes

[teddy7171]

tianzhu:

teddy7171:

Plz help.


Mrs Tan bought 4 times as many toy guns as teddy bears. She spent $1750 altogether. A toy gun cost $10 less than a teddy bears. The total cost of Toy guns was $490 more than the total cost of teddy bears.
i) how much did Mrs Tan spend on the teddy bears?
ii) how much did one teddy bear cost?

Thanks.

Hi

A quick answer.

Cost of Teddy bears ------ (1750 - 490) / 2 ------ 630

Cost of Toy guns ------ 1120


Toy guns ------ 4units

Teddy bears ------ 1unit

Cost of 1unit of Toy guns ------- 1120 / 4 ------ 280

Cost of 1unit of Teddy bears ------- 630

630 - 280 ------- 350

Difference in cost between 1 TG and 1 TB ------- 10

Number of Teddy Bears ------ 350/10 ------- 35

Cost per Teddy bear ------- 630 / 35 ------ 18

Best wishes

Tianzhu,

Thank u.

[Jcong]

The number of the marbles in Box A, Box B and Box C was 195. John added 60 marbles to those in Box A, doubled the number of marbles in Box B and halved the number of marbles in Box C. The ratio of the number of marbles become 4:1:2. What is the total number of marbles in the three boxes now?

[MathIzzFun]

Jcong:

The number of the marbles in Box A, Box B and Box C was 195. John added 60 marbles to those in Box A, doubled the number of marbles in Box B and halved the number of marbles in Box C. The ratio of the number of marbles become 4:1:2. What is the total number of marbles in the three boxes now?

Hi

In the end, the ratio of marbles in box A, B, C -
A : B : C
= 4 : 1 : 2
= 8u : 2u : 4u >> total 14u

Initially,
A : B : C
= 8u - 60 : u : 8u

So, 8u - 60 + u + 8u = 195
17u = 255
1u = 15

14u = 14 x 15 = 210

There are 210 marbles in all the boxes now.


cheers.

[BeContented]

Hi,

Need some help to solve this question. I believe I have the solution, just dunno how to use the model method to show/explain to DS. Is there a need to use model actually? Thanks & happy holiday

Question:
The ratio of the number of red counters to the number of blue counters in a box is 7 : 3. If 36 red counters are removed and 24 blue counters added, there will be an equal number of red counters and blue counters. How many red counters are there in the box?

[MathIzzFun]

cwc:

Hi,

Need some help to solve this question. I believe I have the solution, just dunno how to use the model method to show/explain to DS. Is there a need to use model actually? Thanks & happy holiday

Question:
The ratio of the number of red counters to the number of blue counters in a box is 7 : 3. If 36 red counters are removed and 24 blue counters added, there will be an equal number of red counters and blue counters. How many red counters are there in the box?

Hi

Hope this helps :lol:

http://www.flickr.com/photos/62167097@N02/5674641247/in/photostream

cheers.

[BeContented]

MathIzzFun:

cwc:

Hi,

Need some help to solve this question. I believe I have the solution, just dunno how to use the model method to show/explain to DS. Is there a need to use model actually? Thanks & happy holiday

Question:
The ratio of the number of red counters to the number of blue counters in a box is 7 : 3. If 36 red counters are removed and 24 blue counters added, there will be an equal number of red counters and blue counters. How many red counters are there in the box?

Hi
Hope this helps :lol:

http://www.flickr.com/photos/62167097@N02/5674641247/in/photostream

cheers.

Hi,
Thanks for the help. So I see, I need to draw the model before and after, thanks.

One more question, How many red counters are there in the box? Should I be looking at the red counters before 36 was removed or after it's being removed. I found this a little vague.