Q&A - PSLE Math

[tianzhu]

Essential:

Need help, please kindly assist. At 11am a car left town P for Town Q. Travelling at a speed of 80 km/h. At the same time , another car left town Q for town P at a speed of 100 km / h . The distance between the two towns was 720 km.at what time did the cars pass each other ?

Hi

Good Morning.

As usual for Speed questions, start with a simple diagram.

In 1h, the two cars covered 180 km (100 + 80).

When they pass each other or meet, they cover the entire distance between P and Q which is 720 km.

720/180 ------- 4 h

Hence, they passed each other at 3 pm.

Alternatively, if students are familiar with inverse proportion, they may also use it to calculate the distance travelled by the two cars.

Speed ratio

Car A: Car B ------- 80:100 ------ 4.5

The distance ratio remains the same as 4:5

9 units ------ 720

1 unit ------ 80

Distance covered by car A ------ 4*80 -------- 320 km

Time taken by car A ------- 320/80 -------- 4h

You may also use the distance covered by Car B and get the same answer of 4h.

In this case,(5*80)/100 ------ 4h

Best wishes

[Essential]

Good morning Tianzu, thank u very much for helping …

[Michaelia0816]

Pls help with this questions,


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?

My method:
Girls—1u+4
Boys—1u
In the class—2u+4
Half of the class—1u+2

1-1/3=2/3
1-7/11=4/11
(Take pubic bus) 1u+2-----1/3(boys)+7/11(girls)
(Do not take pubic bus) 1u+2-----2/3( boys)+4/11( girls)
Then, I dun know how to continue the next step, so help…

2) Isa had a total of 1464 orange, yellow and purple balloons for sale. The ratio o the orange balloons to the number of yellow balloons was 2:3. After Isa sold 3/4 of the yellow balloons, 1/2 half of the purple balloons and none of the orange balloons, he had 812 balloons left. How many yellow balloons did Isa have at first?

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.

[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