Q&A - PSLE Math

[tianzhu]

Michaelia0816:

1) In a class, there are 4 more girls than boys. 1/3 of the boys and 7/11 of the girls take pubic buses to school. If 1/2 of the class takes pubic bus to school, how many pupils are there in the class?

Hi

You may use “Units and Parts”.

Boys

Total number ----- 3 units

1 unit ----- take public bus

Girls

Total number ----- 3 units + 4

1/2 of the class takes pubic bus to school

Total number of boys and girls in class ----- 6 units + 4

1/2 of the class ----- 3 units + 2

Hence, number of girls taking public transport ------ 2 units + 2 (3 units + 2 – 1 unit)

If you need a diagram at this stage, draw a simple one.

2 units + 2 ----- 7 parts (7/11)

1 unit + 2 ----- 4 parts (4/11)

Equalise the parts or cross multiply.

8 units + 8 ----- 7 units + 14

1 unit ----- 6

Number of girls ----- 3*6 + 4 ----- 22

Number of boys ----- 18

Total number ----- 40

Another possible way is to use logical reasoning with “SL”.

We know that the number of girls must be multiples of 11 and the average class size is about 30/40.

Work out a listing and see that the conditions are met.

Best wishes.

[tianzhu]

Michaelia0816:

My answer for this is 960, but I do checking, I found out it is wrong so I really really dun know do this question.

Hi

If you work based on the question, you would get an answer of 960.

However when you plugged back to check, it defied logic.

Please check your question.

Best wishes

[tech_nomax]

Pls help with these questions:

1) In March, the number of goldfish in an aquarium was 2/5 the number of angelfish. In April, some goldfish gave birth and its population increased by 80. In the same month, after 50 angelfish died, there were 80% as many goldfish as angelfish. What was the difference between the number of goldfish and the number of angelfish left in the aquarium?

2) At a farm, there were 360 more chickens than ducks. 3/5 of the chickens and 25% of the ducks were sold. In the end, there were 80 more ducks than chickens. How many chickens were there at the farm in the end?

[tianzhu]

tech_nomax:

2) At a farm, there were 360 more chickens than ducks. 3/5 of the chickens and 25% of the ducks were sold. In the end, there were 80 more ducks than chickens. How many chickens were there at the farm in the end?

Hi

You may use MD or work using units.

Chickens ------ 20 units + 360

Chickens@sold ----- 12 units + 216 (3/5)

Chickens@left ----- 8 units + 144

Ducks ------ 20 units

Ducks@sold ----- 5 units (25%)

Ducks@left ----- 15 units

15 units ----- 8 units + 144 + 80

1 unit ----- 32

Number of chickens in the end ------ 8 units + 144 ------ 400

Best wishes

[tianzhu]

tech_nomax:

Pls help with these questions:

1) In March, the number of goldfish in an aquarium was 2/5 the number of angelfish. In April, some goldfish gave birth and its population increased by 80. In the same month, after month, after 50 angelfish died, there were 80% as many goldfish as angelfish. What was the difference between the number of goldfish and the number of angelfish left in the aquarium?

Hi

Please check phrase highlighted in pink.

Best wishes

[tech_nomax]

tianzhu:

tech_nomax:

Pls help with these questions:

1) In March, the number of goldfish in an aquarium was 2/5 the number of angelfish. In April, some goldfish gave birth and its population increased by 80. In the same month, after month, after 50 angelfish died, there were 80% as many goldfish as angelfish. What was the difference between the number of goldfish and the number of angelfish left in the aquarium?

Hi

Please check phrase highlighted in pink.

Best wishes

Hi Tianzhu, thanks for helping.

1) In March, the number of goldfish in an aquarium was 2/5 the number of angelfish. In April, some goldfish gave birth and its population increased by 80. In the same month, after 50 angelfish died, there were 80% as many goldfish as angelfish. What was the difference between the number of goldfish and the number of angelfish left in the aquarium?

[tianzhu]

tech_nomax:

Hi Tianzhu, thanks for helping.

1) In March, the number of goldfish in an aquarium was 2/5 the number of angelfish. In April, some goldfish gave birth and its population increased by 80. In the same month, after 50 angelfish died, there were 80% as many goldfish as angelfish. What was the difference between the number of goldfish and the number of angelfish left in the aquarium?

Hi

You're welcome.

You may use “Units and Parts”.

In March

Goldfish ------ 2 units

Angelfish ------ 5 units

In April, some goldfish gave birth and its population increased by 80. In the same month, 50 angelfish died.

Goldfish ------ 2 units + 80

Angelfish ------ 5 units – 50

2 units + 80 ------ 4 parts (80%)

(x5)
10 units + 400 ----- 20 parts

5 units – 50 ------ 5 parts (100%)

(x4)
20 units – 200 ----- 20 parts

Equalise the parts.

10 units + 400 ------ 20 units – 200

10 units ----- 600

1unit ----- 60

Number of goldfish left ------ 2 units + 80 ------ 200

Number of angelfish left ------ 5 units – 50 ------ 250

Difference ------ 50

Best wishes

[its888me]

speedmaths.com:

its888me:

[quote=\"its888me\"]So sorry, another question from Nanyang prelim 2010 again, Q11:


Diagram A shows a closed rectangular tank of dimensions 20cm by 20cm by 40cm with a cube of edge 10cm glued to its bottom left corner. The tank contains 12 litres of water.

Diagram B shows the same rectangular tank lying on its side. Find the height of the water level, in cm, corrected to 2 decimal places.

http://i48.tinypic.com/302o09z.jpg\">

Again, TIA!

Can someone help with this question? My child assumed that in the Diagram B, the water level reached at the 1/2 point at the cube, thus, getting the answer 14.36cm. Given answer is 15.71.

Again, thanks a lot!

Hi,

Let's try this method:

First,
find the \"Volume of Air\" at top part of Diagram A.
(Vol of air) = (Vol of tank) - (Vol of water) - (Vol of cube).

Next,
find height (or depth) of air at top of Diagram B.
(Height of air) = (Vol of air) / (nett Base Area)
nett Base Area = (40 x 20) - (10 x 10)

Finally,
Height of water = 20 cm - (height of air)

Can you work from here?

Hope this helps.

Cheers

speedmaths.com


.[/quote]Thanks speedmaths.com! Will try to work it out. Many thanks!

[Recia Lim]

Please help to solve this speed question. Thanks!

Question: Mrs Tan took 40 min to walk from her home to the market. On her return journey, she took an additional 8 min. her speed on the way back was 1km/h slower. What was the distance between her home and the market?

[Michaelia0816]

tianzhu:

Michaelia0816:

My answer for this is 960, but I do checking, I found out it is wrong so I really really dun know do this question.

Hi

If you work based on the question, you would get an answer of 960.

However when you plugged back to check, it defied logic.

Please check your question.

Best wishes

Hi! Thx you for helping my pervious question, I decided to use SL for it as I not really used to Units and Parts question, but I will try to use more of it often later on since it is common on hard question. But I still cannot get the balloons question right, can you teach me?
:thankyou: