Q&A - PSLE Math

[ozora]

MathIzzzFun:

ozora:

Got this question:

The ratio of the number of pupils in class A to the number of pupils in class B is 5: 4 at first. 1/5 of the pupils in class A and 7/8 of those in class B were girls. After reshuffling of pupils between 2 classes, the number of boys in each class is equal to the number of girls. If the total of 24 pupils are affected, how many pupils are there in each class at first?
I got the answer class A-42 and B-30 which are wrong.
Anyone can help?
Answer is class A-40 and B-32

Hi

a model approach

http://www.flickr.com/photos/62167097@N02/5834499397/in/photostream

thanks MathIzzzFun
cheers.

[ozora]

MathIzzzFun:

ozora:

Got this question:

The ratio of the number of pupils in class A to the number of pupils in class B is 5: 4 at first. 1/5 of the pupils in class A and 7/8 of those in class B were girls. After reshuffling of pupils between 2 classes, the number of boys in each class is equal to the number of girls. If the total of 24 pupils are affected, how many pupils are there in each class at first?
I got the answer class A-42 and B-30 which are wrong.
Anyone can help?
Answer is class A-40 and B-32

Hi

a model approach

http://www.flickr.com/photos/62167097@N02/5834499397/in/photostream

cheers.

thanks but why is it cut into 4 units for the class A and 2 units each for class B?

[MathIzzzFun]

ozora:

MathIzzzFun:

[quote=\"ozora\"]Got this question:

The ratio of the number of pupils in class A to the number of pupils in class B is 5: 4 at first. 1/5 of the pupils in class A and 7/8 of those in class B were girls. After reshuffling of pupils between 2 classes, the number of boys in each class is equal to the number of girls. If the total of 24 pupils are affected, how many pupils are there in each class at first?
I got the answer class A-42 and B-30 which are wrong.
Anyone can help?
Answer is class A-40 and B-32

Hi

a model approach

http://www.flickr.com/photos/62167097@N02/5834499397/in/photostream

cheers.

thanks but why is it cut into 4 units for the class A and 2 units each for class B?[/quote]hi

number of students in Class A n Class B in ratio of 5 : 4 = 20 units : 16 units

given also : 1/5 of Class A and 7/8 of Class B are girls. So,

1/5 of 20 units = 4 units
1/8 of 16 units = 2 units

cheers.

[tianzhu]

P5G:

Uncle Tianzhu,

Thank you for your clear explanations. So for the log cake, we are talking about a 'whole' log cake and not just 7/8.

Will the answer be just 1/3 if \"of a\" is removed from the question as follows:
Nomee distributed 7/8 log cake equally among her twin sister and 2 brothers. What fraction of the log cake did each of them receive?

Hi P5G

Good Morning.

Nomee distributed 7/8 of a log cake equally among her twin sister and 2 brothers. What fraction of the log cake did each of them receive?

In this question, having some form of pictorial representation does help one to see the relationship.

This question touches on “Product of Proper Fractions” which is covered in P5.

Students who are familiar with it can write a number sentence 7/8÷3 or 7/8*1/3 and arrive at an answer of 7/24.

If you rewrite the question as

Nomee ate 1/8 of a log cake and distributed the remainder equally among her twin sister and 2 brothers. What fraction of the remaining part of the log cake did each of them receive?

The answer is 1/3.

Usually, you'll be asked to find the fraction of a whole.

Best wishes

[KSP]

Hi need help here. Thanks.


Lily is meeting a friend at a certain time. If she drives at 80km/h, she will be 1/3 hr late. If she drives at 60km/h, she will be 3/4hr late. How long will the journey take if she drives at 90km/h?

[corneyAmber]

KSP:

Hi need help here. Thanks.


Lily is meeting a friend at a certain time. If she drives at 80km/h, she will be 1/3 hr late. If she drives at 60km/h, she will be 3/4hr late. How long will the journey take if she drives at 90km/h?

Yo KSP! Why are you doing PSLE now? :?

[corneyAmber]

Distance = speed x time

80 x (u+1/3) = 60 x (u+3/4)
80u + 80/3 = 60u + 45
20u = (135-80)/3
20u = 55/3
u = 55/60 = 55 mins

1/3 hr = 20 mins
3/4 hr = 45 mins

so distance = 80 x (55/60 + 20/60) = 100km
check: distance = 60 x (55/60 + 45/60) = 100km
So if speed =90km/h
time = 100/90 h = 1 1/9h.

[Dharma]

When distance is fixed; speed and time are inversely proportional.

Speed ratio => 80 : 60 = 4 : 3
Time ratio => 3: 4
4u – 3u = 1u = (3/4 – 1/3) = 5/12 hrs
Total distance = 80 x 3 x 5/12 = 100km
Time taken when speed is 90km/hr = 100/90 = 10/9 hrs

[corneyAmber]

Dharma:

When distance is fixed; speed and time are inversely proportional.

Speed ratio => 80 : 60 = 4 : 3
Time ratio => 3: 4
4u – 3u = 1u = (3/4 – 1/3) = 5/12 hrs
Total distance = 80 x 3 x 5/12 = 100km
Time taken when speed is 90km/hr = 100/90 = 10/9 hrs

I like this shorter approach! :imcool:

[Dharma]

ksi:

Dharma:

When distance is fixed; speed and time are inversely proportional.

Speed ratio => 80 : 60 = 4 : 3
Time ratio => 3: 4
4u – 3u = 1u = (3/4 – 1/3) = 5/12 hrs
Total distance = 80 x 3 x 5/12 = 100km
Time taken when speed is 90km/hr = 100/90 = 10/9 hrs

I like this shorter approach! :imcool:

Glad you like it, ksi.