Q&A - PSLE Math

[AJH]

tianzhu:

AJH:

Hi - how did u get 480? I got 460. Thank u.

Hi

This question is crafted in such a way that it’s neat to use MD.

You may also use Units and Parts or letters of the alphabet to show the variables.

What you are seeing now is already the MD.The boxes are labelled with units and parts to enable students using the UP to see better.

The number “480” refers to the number of girls.

Best wishes

:thankyou: for your help. But isn't 30% of students in the parade ground and 70% of students at the indoor sports hall = 180 + 280 = 460?

[AJH]

tianzhu:

AJH:

Hi - nd help with this question:


Mr Lee packed some oranges into big boxes and apples into small boxes. Each big box contained 50 oranges and each small box contained 30 apples. After packing, there were 12 more big boxes than small boxes. Given that there were 1240 fewer apples than oranges, how many oranges were there? ( ans: 2200 )

Thank you.

Hi

You may use MD or work with units.

Draw three boxes (stacked up) to show the number of apples.

12*50 ------ 600

Draw five boxes to show the number of oranges. To make the MD look neater, divide 600/5 to give 120, add a bar showing 120 to each of the box.

5 boxes + 600 ------- 3 boxes + 1240

2 boxes ------- 640

1box ------ 320

Number of apples ------- 2200

Best wishes

:thankyou: I understand now.

[imacsg]

Hi MathIzzzFun,

Thanks for your prompt reply.

I know how to do part B. Thanks alot.

[tianzhu]

AJH:

tianzhu:

But isn't 30% of students in the parade ground and 70% of students at the indoor sports hall = 180 + 280 = 460?

Hi

Good Morning.

The number of boys and girls remains the same before and after the transfer. The total number of 1000 did not change at all.

The number of girls stays the same that is 480 before and after.

In this case

Number of girls in the beginning ------- 480

Number of girls in the end (after the transfer)

Parade square ------- 3 parts ------ 3*55 ------- 165 (30% of 550)

Indoor stadium -------- 7 units ------- 7*45 ------- 315 (70% of 450)

Total number ------ 165 + 315 ------- 480

Best wishes

[tamunawara]

hi can someone help me with this problem from Hendry Park preliminary examination 2012


Susie wants to buy a handbag with her savings. If she increases her savings by 10% she will need $14 more to buy the handbag. if she increases her saving by 45% she will have $7 is excess. what is the cost of her handbag?

[tianzhu]

tamunawara:

hi can someone help me with this problem from Hendry Park preliminary examination 2012


Susie wants to buy a handbag with her savings. If she increases her savings by 10% she will need $14 more to buy the handbag. if she increases her saving by 45% she will have $7 is excess. what is the cost of her handbag?

Hi

It helps to draw a simple diagram.

This question is similar to the Excess and Shortage type of questions

35% ------ 14 + 7 ------ 21

5% ------- 3

10% ------ 6

110% ------ 66

Cost of handbag ------- 64 + 14 ------- 80

or

145% ------- 87

Cost of handbag ------- 87 - 7 ------- 80

Best wishes

[tamunawara]

thanks a lot

[Dazzle]

Pls help to solve this St. Hilda Prelim Q15

http://i50.tinypic.com/2zyabr8.jpg\">

[Dazzle]

http://i45.tinypic.com/2dvralg.jpg\"> Realised the earlier pic is way too small. see if this is big enough

[MathIzzzFun]

ksmama:

[quote]

MathIzzzFun:

After completing 2/7 of the journey in 1/2 hour, the car passed a van travelling at a constant speed of 70km/h in the same direction.
3 hours later, the car reached Town Q but the van was still 65 km away from Town Q.

(a) Find the average speed of the car.

(b) If the van left Town P at 0810, at what time would it arrive at Town Q?



5/7 of distance PQ --> 3 x 70 + 65 = 275 km
distance PQ --> 275/5 x 7 --> 385 km
total time taken by car --> 3. 5 h
average speed of car --> 385/3. 5 --> 110 km/h

cheers.

Hi mathsizzfun,
Regardg yr ans to the foll questn, my kid calculated a diff way n the ans is different. Please comment on why his method is not acceptable. Is it because when d ques asks for average sp, we have to calculate based on whole dist n time taken n not assume aver sp same throughout? Thanks.

Diff in speed-> 65/3 =21.67km/h
Speed of car 70+21.67 = 91.67km/h.

[/quote]It is incorrect to find average speed by finding the average of averages ie average = 1/2 x (average 1 + average 2)

The definition of average speed is
Average speed = (total distance travelled)/(Total time taken)

For eg. Mr Tan travelled 20 km at 40km/h and another 20 km at 60km/h, what is his average speed ?
It is incorrect to calculate average speed = 1/2 x (40+60)

The right way is to find the total time taken --> 20/40 h + 20/60 h = 5/6 h
average speed = (total distance travelled)/(total time taken) = 40 ÷ 5/6 = 48 km/h

cheers.