Q&A - PSLE Math

[AnnT2009]

Dharma, Tianzhu and edanson,


Thank U.

[Dharma]

Thanks for posting Fig 2 of Q1, tianzhu… makes it all the more obvious

[tianzhu]

edanson:

TianZhu - are you a school teacher or Maths tutor or a lover for :love: Maths too ? You are great and extremely clear in your explaination. You are also very IT savy to be able to put your explainatione in pictures.

Any hint how you do that, wanted to learn to help my kids too :oops: .

Hi edanson

All your guesses were wrong. I am a victim of circumstances. Private schools specialising in Maths are expensive and my pockets are not deep enough to send my son there. Realising that he needs help at certain times, I am forced to read up so as to be able to help him when he needs it.

I am glad you liked my solutions. I want to show my son that Maths is beautiful. It can be colourful and fun too.

I am definitely not IT savvy, just a one finger typist. I use PowerPoint 2007 to work out the answers.

[tianzhu]

Dharma:

Thanks for posting Fig 2 of Q1, tianzhu..... makes it all the more obvious

Hi
You may view the questions @ http://www.misskoh.com

[Dharma]

Hi Friends


Came across a question posted earlier …The Xmas Tree shape Qn …wish to share a different approach to solve this qn using patterns.

a)
If we notice the centre column of numbers …their difference are all in multiples of 4.

We can determine the middle column nos. as follows

Row 1 = 1 [1+0] = [1+4(0)]
Row 2 =5 [1+4] = [1+4(1)] [1+4(1)]
Row 3 =13 [1+4+8] = [1+4(1)+4(2)] [1+4(1+2)]
Row 4 = 25 [1+4+8+12] = [1+4(1)+4(2)+4(3)] [1+4(1+2+3)]
.
.
Row N [1+4(1)+4(2)+…4(N-1)] [1+4(1+2+…+(N-1)]

We can therefore determine the middle number of Row 6 by using the pattern above

Middle number of row 6 - [1+4(1+2+3+4+5)] = 61

We now need to find out how many numbers are there in each row…

Row Pattern
1 - 1
2 - 3 [(2x2)-1]
3 - 5 [(3x2)-1]
. .
. .
N 2N-1

There are 2(6)-1 = 11 numbers in Row 6[ 5 numbers on the left and 5 numbers on the right of the middle number]

3rd number of Row 6 = 61 - (5-2)x2
= 55

b) To find the sum of numbers in the 1st 5 rows (all odd numbers from 1 to 49)

Sum of all numbers from 1 to 49 = (49x50)/2
= 1225

Sum of all even numbers from 1 to 49 = 2(1+2+3+ …+22+24)
= 2 x (24x25)/2
= 600

Sum of all odd numbers from 1 to 49 = 1225 - 600
= 625

Hope that this explanation is useful …

[small]

chengsmummy:

Hi,


Q1, the answer should not be 15m, because Linus and Gereth are travelling on the different speed, so the difference should not remain the same.
assume they took 1 unit of time to complete the first 85 and 70m respectively, then Gereth will take 15/85 unit time to finish the remaining 15m.
then Linus will travel 15/85 * 70 = 12.35m at the same time. so when Gereth at the finish line, Linus was 30-12.35 = 17.65m away.

tianzhu:

Hi, hope this helps.

Q1. Linus was 15m away. Difference between Gareth and Linus was (30-15).

Q2. Length of one side of equilateral triangle = 12
Area of triangle CYW = ½*6.8*3 = 10.2 (XW=3, midpoint)
Area of triangle CXW = 25.8 – 10.2 = 15.6 cm2

hi chengsmummy and tianzhu,

Thanks for your help..

[small]

Thanks for your time again.. :oops:


Question:
There were 180 tomatoes and carrots in a basket at first. 2/5 of the tomatoes and 2/3 of the catrrots were used for cooking. In the end, there were 80 tomatoes and carrots left in the basket. How many:

a) tomatoes were there at first?
b) carrots were used in the coooking?

I only knew the algebra method.

[corneyAmber]

http://i724.photobucket.com/albums/ww246/ks2me/P6Mathtomatoes-carrots-1.jpg\">

[Dharma]

Hi,


You may wish to solve the question using fractions and simultaneous equations. Simultaneous equations are taught in secondary schools but many primary school kids are very familiar with them ....problem can be solved quickly ....thus allowing the child more time for challenging questions

a)
5/5T + 3/3C = 180 - Eqn 1
3/5T +1/3C = 80 - Eqn 2

Multiply Eqn 1 by 3

9/5T + 3/3C = 240 - Eqn 3

Eqn 3 minus Eqn 1

4/5T = 240 - 180 = 60

No. of tomatoes at first - 75

b)
No. of carrots at first - 180 -75 = 105
No. of carrots used for cooking = 2/3 x 105 = 70

[Dharma]

A question quite similar to this appeared in PSLE 2008 Maths paper


A millipede crawled up a slippery wall.
For every 3cm that it crawled, it slipped back by 1cm.
It took 3 seconds to crawl 1 cm and 1 second to slip back 1cm.

a) What was the height the millipede climbed in 1 minute? [1]
b) What was the shortest time needed for the millipede to climb 56cm? [3]

A non-routine type of question …