Q&A - PSLE Math

[tianzhu]

the shadowed snake:

WHY??????? :? :? :? :? :? :? is it like this????

Hi

As given in the question, when George completed 3/4 of the journey, Hamid was 18km behind George.

So when George completed 1 /4 or 1 part of the journey, Hamid was 6 km behind George, when George completed 1/2 or 2 parts of the journey, Hamid was 12 km behind George.

Therefore, when George completed the whole journey, Hamid was 24 km behind George.

Best wishes

[PapayaDad]

tianzhu:

the shadowed snake:

WHY??????? :? :? :? :? :? :? is it like this????

Hi

As given in the question, when George completed 3/4 of the journey, Hamid was 18km behind George.

So when George completed 1 /4 or 1 part of the journey, Hamid was 6 km behind George, when George completed 1/2 or 2 parts of the journey, Hamid was 12 km behind George.

Therefore, when George completed the whole journey, Hamid was 24 km behind George.

Best wishes

and so hamid took 20 minutes to cover 24km
his speed is 24 x 3 = 72km/h
(change 20 minute to 1 hour by 20 x 3 = 60 minutes ; so distance also multiply 3)

[mathnoobs]

MathIzzzFun:

http://i40.tinypic.com/ztj18k.png\">

looks like both are cyclists and Bala is certainly a pro-racer.

cheers.

Hi MathIzzzFun
I’m afraid I don’t quite understand how to simplify the following expression:
(18 units – 60km)/9units x 18
In your example, this will simplify to 36 units – 120 km
I’m stuck at how to eliminate the 9units from the denominator.

thanks

[tianzhu]

Mary Joy:

please help me with these speed questions


1.Mr Tan travelled from town A to town b.he took 2.5 hours to travel the first 156 km and 3/5 of that time to travel the remaining 2/5 of the journey.what was his average speed for the whole journey?

Hi

Good Morning.

It’s helpful to start with a simple diagram.

Draw a line with 3 parts equal to 156 km. Hence, 1 part ------ 52km.

Mr Tan took 2.5h to cover 156 km.

3/5 of 2.5 h ------ 1.5 h

remaining 2/5 of the journey -------- 2*52 ------- 114 km

Total distance -------- 260 (156+114)

Total time ------- 4 (2.5+1.5)

Average speed -------- 260/4 ------ 65 km/h

Best wishes

[tianzhu]

Mary Joy:

2.susan drove from city mall to parklane mall at an average speed of 80 km/h.Danny left city mall one hour earlier than Susan and took 5 hours to reach parklane mall,traveling at an average speed of 72 km/ h.danny reached parklane mall at 8 pm.
a) at what time did Susan leave city mall?
b) how far was Susan from parklane mall when Danny reached the destination?

Hi

Start with simple diagram.

Distance between CM and PM ------ 5*72 ------ 360 km

Danny took 5h to reach PM. He reached PM at 8pm.Draw a timeline and work backwards; Danny started his journey at 3pm.

a)Hence, Susan started at 4pm (Danny left city mall one hour earlier than Susan)

b)Distance covered by Susan in 4h (from 4pm to 8pm) ------ 4*80 -------- 320km

360 – 320 ------- 40

Susan was 40km away from PM when when Danny reached the destination

Best wishes

[MathIzzzFun]

mathnoobs:

Hi MathIzzzFun
I’m afraid I don’t quite understand how to simplify the following expression:
(18 units – 60km)/9units x 18
In your example, this will simplify to 36 units – 120 km
I’m stuck at how to eliminate the 9units from the denominator.

thanks

oops ... you are right..thanks for highlighting the error.. my apologies.
For this case, there will be no solution as the resultant quadratic equation will yield no real roots.

In order to get a real value for the speed, the question should be amended as follows:

Mr Tan travelled from Town A to Town B at 11:00. When he was 18 km from Town A , Mr Bala left Town B to Town A at a speed of 18km/h faster than Mr Tan. When Mr Bala reached Town A at 13:00, Mr Tan had completed 90 percent of his journey. What was Mr Tan's average speed?

in this case,
Mr Tan's speed --> 54 km/h and
Bala's speed --> 72 km/h

cheers.

[Mary Joy]

tianzhu:

Mary Joy:

please help me with these speed questions


1.Mr Tan travelled from town A to town b.he took 2.5 hours to travel the first 156 km and 3/5 of that time to travel the remaining 2/5 of the journey.what was his average speed for the whole journey?

Hi

Good Morning.

It’s helpful to start with a simple diagram.

Draw a line with 3 parts equal to 156 km. Hence, 1 part ------ 52km.

Mr Tan took 2.5h to cover 156 km.

3/5 of 2.5 h ------ 1.5 h

remaining 2/5 of the journey -------- 2*52 ------- 114 km

Total distance -------- 260 (156+114)

Total time ------- 4 (2.5+1.5)

Average speed -------- 260/4 ------ 65 km/h

Best wishes

Hi,thanks for your explanation.However,I do not understand why 3 parts is 156 km.If you can,please send a reply explaining why.

:thankyou:

[tianzhu]

Mary Joy:

Hi,thanks for your explanation.However,I do not understand why 3 parts is 156 km.If you can,please send a reply explaining why.

:thankyou:

Hi

The hint lies here “remaining 2/5 of the journey”.

The first 156 km refers to the first 3/5 of the journey.

Best wishes

[Mary Joy]

tianzhu:

Mary Joy:

Hi,thanks for your explanation.However,I do not understand why 3 parts is 156 km.If you can,please send a reply explaining why.

:thankyou:

Hi

The hint lies here “remaining 2/5 of the journey”.

The first 156 km refers to the first 3/5 of the journey.

Best wishes

hi,

Thank you so much for your detailed explanation. :thankyou:

[Emelyn]

Hi,


Need help with the following questions from Nan Hua 2012 SA1 Maths :

Qn13 :
80% of the spectators at a stadium were adults. 75% of the children at the stadium were boys. There were 36 more boys than girls.
(a) How many boys were there at the stadium ?
(b) Some boys left the stadium and after which 10% of the remaining people at the stadium were boys. How many boys left the stadium ?

Qn17 :
In a class library, there are some fiction and non-fiction books. If we remove 1 non-fiction book from the library, 1/7 of the remaining books are non-fiction books. If, instead of removing 1 non-fiction book, we remove 2 fiction books from the library, then 1/5 of the remaining books are non-fiction books. How many books are there in the class library ?

Many many thanks.