Q&A - PSLE Math

[MathIzzzFun]

Jazpy:

Qn: At first, Kumar had only $5-notes and Devi had only $2-notes. The number of notes Kumar had is 2/3 of Devi's notes. When Devi gave Kumar $80, he changed all the $80 to $5 notes. Then the number of notes Devi has now is 10% more than Kumar. How many notes does Devi have at the end????



Please help

Hi

At first,
Number of $5 notes : Number of $2 notes = 2 : 3
Value of $5 notes : Value of $2 notes = 2 x 5 : 3 x 2 = 10 : 6 = 5 : 3

In the end,
number of $5 notes : number of $2 notes = 10 : 11
Value of $5 notes : Value of $2 notes = 10 x 5 : 11 x 2 = 50 : 22

Total value remained the same,
At first, Value of $5 notes : Value of $2 notes = 5 : 3 = 45u : 27u (total 72u)
In the end, Value of $5 notes : Value of $2 notes = 50u : 22u (total 72u)

So, 50u – 45u = 5u = $80, 1u = $16

Number of $2 notes in the end = 22u /($2) = 22 x 16 / 2 = 176

alternative approach using MD - http://www.flickr.com/photos/62167097@N02/6010152839/in/photostream/

cheers.

[matrix0405]

Math models: Using Before & After approach + Percentage.


http://www.youtube.com/watch?v=_f2enjbMrLA

[stary]

atutor2001:

stary:

Hi all,


Can someone explain/show me the workings of this question?

http://www.flickr.com/photos/64113082@N03/6007585363/in/photostream
Ans: 5
(SCGS Prelim 2010 P6 Question 3)

Thanks!

Add the numbers at the 4 corners and divide by 4. (the middle number is the average)

Thanks a lot!

[stary]

matrix0405:

Math models: Using Before & After approach + Percentage.


http://www.youtube.com/watch?v=_f2enjbMrLA

:goodpost: :goodpost:

[stary]

Hi all,


Have a question to ask u all...

There were some fruits in a warehouse. 3/7 of them were apples and the rest were oranges. After throwing away 27 apples and 1/4 of the oranges that were rotten, there were 3/5 of the fruits left.
(a) How many fruits were thrown away?
(b) How many apples were there at first?

:?

[atutor2001]

stary:

Hi all,


Have a question to ask u all...

There were some fruits in a warehouse. 3/7 of them were apples and the rest were oranges. After throwing away 27 apples and 1/4 of the oranges that were rotten, there were 3/5 of the fruits left.
(a) How many fruits were thrown away?
(b) How many apples were there at first?

:?

Using ratio to solve :

Apple : Orange : Total = 3 : 4 : 7 (Before)
Because the fruits left is given as 3/5 of \"Before\", the total of \"before\" must be divisible by 5. So x 5 to the ratio

Apple : Orange : Total = 15U : 20U : 35U (Before)
Apple : Orange : Total = -27 : -5U : (??) (Fruits thrown away)
Apple : Orange : Total = (15U-27) : 15U : 21U (After)

The decrease in the total is 35U - 21U = 14U
The fruits thrown away is 27+5U

Therefore, 14U = 27 + 5U
9U = 27
U = 3

Total fruits thrown away is 14U = 42
Total apples (at first) is 15U = 45

[stary]

atutor2001:

stary:

Hi all,


Have a question to ask u all...

There were some fruits in a warehouse. 3/7 of them were apples and the rest were oranges. After throwing away 27 apples and 1/4 of the oranges that were rotten, there were 3/5 of the fruits left.
(a) How many fruits were thrown away?
(b) How many apples were there at first?

:?

Using ratio to solve :

Apple : Orange : Total = 3 : 4 : 7 (Before)
Because the fruits left is given as 3/5 of \"Before\", the total of \"before\" must be divisible by 5. So x 5 to the ratio

Apple : Orange : Total = 15U : 20U : 35U (Before)
Apple : Orange : Total = -27 : -5U : (??) (Fruits thrown away)
Apple : Orange : Total = (15U-27) : 15U : 21U (After)

The decrease in the total is 35U - 21U = 14U
The fruits thrown away is 27+5U

Therefore, 14U = 27 + 5U
9U = 27
U = 3

Total fruits thrown away is 14U = 42
Total apples (at first) is 15U = 45

Thanks! :thankyou:

[Saralala]

tianzhu:

1) There were some marbles at a shop. The ratio of the number of red marbles to the number of blue marbles was 2:3. When 50 more red marbles and 30 more blue marbles were added, the ratio of the number of red marbles to the number of blue marbles became 5:6. How many marbles were there at first?

Let me try to solve the first question:

Method 1:

If you use algebra, it will be rather fast (trained my son to do so as PSLE still allows):

2x + 50 = (5/6) (3x +30)
x = 50

5 units at the start so 50 x 5 = 250 marbles at first.

-----------------------------------------------------------------
Method 2 (Diagrams):

Draw 2 units for red and 3 units for blue.

For red, draw dotted unit for 50 extra red and a 1 unit size to fill up 5:6 to become 6:6.

For blue, just add 30 extra blue as dotted unit.

Thus, 30 + 1 big unit = 50 + 1 small unit

We can say 1 big unit - 1 small unit = 20.

--- Look at red units now. There are 2 big units and 1 50 dotted unit. So 2 small units + 20 + 20 +50 = 5 small units.

3 small units = 90
1 small unit = 30

2 big unit = 30 x 5 - 50
= 100

5 big units = 100/ 2 * 5
= 250.

Alright, sorry if it sounds very difficult, but I don't think it is if I can draw. Haha. Too lazy to draw. :x

[Hifive]

Hi all,


Need help with this question:

During a basketball match, the teacher promised the team of 8 pupils an equal playing time during the 40 minutes match. Given that only 5 players can play at any one time, what is the average playing time for each pupil?

Thanks in advance.

[MathIzzzFun]

Hifive:

Hi all,


Need help with this question:

During a basketball match, the teacher promised the team of 8 pupils an equal playing time during the 40 minutes match. Given that only 5 players can play at any one time, what is the average playing time for each pupil?

Thanks in advance.

Hi

Divide 40 minutes into 8 x 5 minutes time slots, and for each 5 min slot, there will be 5 playing slots, total 5 x 8 playing slots. So, each will get 5 playing slots ie 5 x 5 = 25 min.

http://i55.tinypic.com/24cd3r4.jpg\">

cheers.