Q&A - PSLE Math

[Daddy]

Hi all,


I have 2 ratio questions that I do not understand how to solve them. Any way or method to understand or solve most of the ration questions. Thanks

Here are the questions. Maybe they are easy question.

1. The ratio of the number of boys to the number of girls was 4:9.
When 17 girls left, the ratio of the number of boys to the number of girls became 6:5. How many children were there at first?

2. English exam, 5/7 of Nicholas' marks was twice of Jonathan's marks.
What is the ratio of Niholas' marks to Jonathan's marks?

Thanks
Daddy

[MathIzzzFun]

Daddy:

Hi all,


I have 2 ratio questions that I do not understand how to solve them. Any way or method to understand or solve most of the ration questions. Thanks

Here are the questions. Maybe they are easy question.

1. The ratio of the number of boys to the number of girls was 4:9.
When 17 girls left, the ratio of the number of boys to the number of girls became 6:5. How many children were there at first?

2. English exam, 5/7 of Nicholas' marks was twice of Jonathan's marks.
What is the ratio of Niholas' marks to Jonathan's marks?

Thanks
Daddy

Hi

Q1.
there is no change to the number of boys, equalize the \"share\" for the boys:
ratio of boys to girls, before = 4: 9 = 12u:27u
ratio of boys to girls, after = 6:5 = 12u:10u
So, 17u = 17, 1u=1
Total number of children at first = 12u+ 27u = 39u = 39

Q2. Jonathan marks = 1/2 x 5/7 of Nicholas marks = 5/14 Nicholas marks
So ratio Nicholas's marks : Jonathan's marks = 14: 5

cheers.

[Verysuperkiasu]

firefly38:

DS says teacher told them must reduce fractions to lowest form, even if question does not specify. DS is from GEP but his friends from mainstream are taught that way too.

Lucky I check with u guys. Will ask my son to do that.... Hope he remembers.... Thank You!!!

[Daddy]

Thanks MathIzzzFun,



http://i55.tinypic.com/181a15.jpg\">
I do not understand that \" ratio Men to Women = 8:2 -> 8u:2u.
You added \"u\" to the values.

After 55 men left and 75 women added. new ration = 6:4 = 3:2.
How come you did not add \"u\" to this value? :?
Why? Thanks..

[MathIzzzFun]

Daddy:

Thanks MathIzzzFun,



http://i55.tinypic.com/181a15.jpg\">
I do not understand that \" ratio Men to Women = 8:2 -> 8u:2u.
You added \"u\" to the values.

After 55 men left and 75 women added. new ration = 6:4 = 3:2.
How come you did not add \"u\" to this value? :?
Why? Thanks..

Hi

initial ratio men to women, 8:2 - we can represent it using model drawing ie 8 blocks for men and 2 blocks for women.. so we have 8 units and 2 units..

after 55 men, and 75 women joined.. we have a new situation and the new ratio is 3 : 2 which can be represented in model drawing by 3 blocks of DIFFERENT size than the previous model for men and 2 blocks for women... so we cannot use the same unit block as the first model - refer to the solution and you will understand better.

cheers.

[ozora]

tianzhu:

Hi ozora


Good Morning.

Q1 is a tweaked version of a question from PSLE Maths 2009.
You may solve it using MD or UM.

http://farm7.static.flickr.com/6188/6119123632_b0a9875903_z.jpg\">


For more practice, you may also change the question, instead of Jack buying another 25 apples, change it to Jack buying another 25 oranges.

Mac bought some apples and gave half to Jack.
Jack bought some oranges and gave half to Mac.
Mac threw away 25 rotten oranges and Jack bought another 25 oranges for himself.
In the end, the number of oranges to the number of apples Mac had was 1:4 and the number of oranges to the number of apples Jack had was 1: 2.
How many oranges did Jack buy?


As for Q2, you may use MD or UM to do it.
Not much time, so will share some pointers.

Divide a bar into 15 units.

Black buttons ----- 9units
Black buttons sold ----- 5 units.
Black buttons left ------ 4units

White buttons ------ 6units
White buttons sold ----- 120
White buttons left ------ 6units -120

He had 2/5 of the buttons left.
2/5*15 ------ 6units

6units ------- 4units + (6units - 120)
4units ------ 120
1unit ------ 30

He sold 150 black buttons (5*30) and 120 white buttons

Hence, altogether 270 buttons

Hope this helps.

Best wishes

Thanks tianzhu. it really helps. It unblocks my mental brain. THanks thanks

[ozora]

may i know how do you search for the previous solutions posted given for similiar questions?

[Despair]

Dear all, I require help with this Speed question.


A truck started its journey from Village Tiku to Village Chuka at 16 30. At the same time, a bus started travelling from Village Chuka to Village Tiku.

The ratio of the truck’s speed to the bus’s speed is 8:3. At 17 00 the two vehicles passed each other. The truck increased its speed and reached Village Chuka at 17 20 while the bus increased its speed by 18km/h.

When the truck reached Village Chuka, the bus is still 40 km away from Village Tiku.

What is the distance between the two villages?

Thank you in advance.

[tianzhu]

Daddy:

Thanks, why you used \"5 x 19 = 95cm\". I do not understand. Thanks.

Hi

Hope this helps.

Best wishes.

http://farm7.static.flickr.com/6068/6119347253_10aca709c9_z.jpg\">

[MathIzzzFun]

Despair:

Dear all, I require help with this Speed question.


A truck started its journey from Village Tiku to Village Chuka at 16 30. At the same time, a bus started travelling from Village Chuka to Village Tiku.

The ratio of the truck's speed to the bus's speed is 8:3. At 17 00 the two vehicles passed each other. The truck increased its speed and reached Village Chuka at 17 20 while the bus increased its speed by 18km/h.

When the truck reached Village Chuka, the bus is still 40 km away from Village Tiku.

What is the distance between the two villages?

Thank you in advance.

Hi

May I know the source of this problem sum? The setter probably adapted the problem by changing the numbers..firstly, after the truck and bus passed each other, the truck reduced its speed rather than increased its speed because if it had continued at original speed, it would hve reached the destination in 3/8 x 30min = 11.25min.

Ratio of distance travelled by truck n bus = 8u : 3u, total distance 11u.

after truck and bus passed each other, truck took 20 min to reach Chuka,and the bus is still 40 km from Tiku.

If bus had continued its original speed, the distance it would travel = 20/30 x 3u =2u, since the bus's speed is increased by 18km/h, it would travel 18x20/60 = 6km more, ie it travelled a distance of 2u + 6km after passing the truck, and it was 40km from Tiku.

Distance from Tiku to the \"meeting point\" = 8u
8u = 2u + 6km + 40km, 6u = 46km, 11u = 11/6 x 46 km = 84 1/3 km
Distance between Tiku and Chuka = 84 1/3 km.

cheers.