Q&A - PSLE Math

[tianzhu]

Hi


I find it interesting, a question about life in a Maths thread.

Some live to its fullest, some stumble and fall. Many go through it without knowing its meaning. Well, what is life? I have no answer.

This catchy and entertaining Mongolian tune appeared from nowhere during a random search this morning.
http://www.youtube.com/watch?v=qItIkK7m0n8&feature=related

While listening to this beautiful tune, you may want to read a speech from PM Lee.

Pursue dreams, but give back to society: PM
http://www.pmo.gov.sg/News/Transcripts/Prime+Minister/Pursue+dreams+but+give+back+to+society+PM.htm

A passage from a speech by Dr Goh when he retired from politics in 1984
'You should regard the present condition of the Republic not as a pinnacle of achievement but as a base from which to scale new heights.'

Best wishes

[liketoeat]

Will question like the following appear in PSLE? Could someone pls help to solve. Thank you.


Find the sum of the digits of the first 100 odd numbers. (Ans = 1000)

[corneyAmber]

liketoeat:

Will question like the following appear in PSLE? Could someone pls help to solve. Thank you.


Find the sum of the digits of the first 100 odd numbers. (Ans = 1000)

First 100 odd numbers --->
01,3,5,7,9,11,13,15,17,19 (10 odd numbers)
21,.........31,..............39 (20 odd numbers)
41,.............51...........59 (30 odd numbers)
:
101
121
141
161
181...........................199 (100 odd numbers)

(1+3+5+7+9) = 25
1*5 + 25 = 30
2*5 + 25 = 35
3*5 + 25 = 40
:
:
:
9*5 + 25 = 70
1*5 + 0*5 + 25 =30
1*5 + 1*5 + 25 = 35
1*5 + 2*5 + 25 = 40
:
:
:
1*5 + 9*5 + 25 = 75

Total = 25 +(30 + 35 + .......+70 + 30 + 35 + ..+ 70)+75
= 25+75 + (450*2)
= 100 + 900
=1000

[tianzhu]

liketoeat:

Will question like the following appear in PSLE? Could someone pls help to solve. Thank you.


Find the sum of the digits of the first 100 odd numbers. (Ans = 1000)

Hi

I'd like to find out if you are talking about the sum of these consecutive odd numbers like
1,3,5,7,9 ...............191,193,195,197,199

I am asking beacause your question shows sum of digits, like 8 is one digit and 88 is two digits.

Usually, the question is asked as

Find the sum of the first 100 odd numbers.

If this is the case, the answer is 10000.

Best wishes

[liketoeat]

tianzhu:

liketoeat:

Will question like the following appear in PSLE? Could someone pls help to solve. Thank you.


Find the sum of the digits of the first 100 odd numbers. (Ans = 1000)

Hi

I'd like to find out if you are talking about the sum of these consecutive odd numbers like
1,3,5,7,9 ...............191,193,195,197,199

I am asking beacause your question shows sum of digits, like 8 is one digit and 88 is two digits.

Usually, the question is asked as

Find the sum of the first 100 odd numbers.

If this is the case, the answer is 10000.

Best wishes

I find this question funny as well, I think its asking about the digits...then it will become very tedious to work out, right? I think it won't come out in PSLE!

[liketoeat]

ksi:

liketoeat:

Will question like the following appear in PSLE? Could someone pls help to solve. Thank you.


Find the sum of the digits of the first 100 odd numbers. (Ans = 1000)

First 100 odd numbers --->
01,3,5,7,9,11,13,15,17,19 (10 odd numbers)
21,.........31,..............39 (20 odd numbers)
41,.............51...........59 (30 odd numbers)
:
101
121
141
161
181...........................199 (100 odd numbers)

(1+3+5+7+9) = 25
1*5 + 25 = 30
2*5 + 25 = 35
3*5 + 25 = 40
:
:
:
9*5 + 25 = 70
1*5 + 0*5 + 25 =30
1*5 + 1*5 + 25 = 35
1*5 + 2*5 + 25 = 40
:
:
:
1*5 + 9*5 + 25 = 75

Total = 25 +(30 + 35 + .......+70 + 30 + 35 + ..+ 70)+75
= 25+75 + (450*2)
= 100 + 900
=1000

Wow, will it come out in PSLE? Thank you.

[corneyAmber]

Actually it is not tedious to work out, there is a pattern to calculate everything quite fast, I present more details for you to see but in exam, this can be done in 4 lines.


Having said this, this is more MO type of questions. However, I notice selective MO type of questions are getting into the PSLE as well.

[sexymama]

I've been struggling

with my DD h'wk....Pls help..

(1) Mr Mohn bought a certain number of cameras and watches for $1980.
Each camera cost $60. Each watch cost 1/4 as much as a camera.
If 40% of the items he bought were cameras, how many watches
did he buy?

(2) $400 were shared by 4 boys. Alan received 25% of the total amount
that Ben, Carl and Dan received. Ben's share was 60% of the
amount received by Carl and Dan. Carl received 4 times as much
as Dan. How much more did Ben receive than Dan?

[tianzhu]

ksi:

Actually it is not tedious to work out, there is a pattern to calculate everything quite fast, I present more details for you to see but in exam, this can be done in 4 lines.


Having said this, this is more MO type of questions. However, I notice selective MO type of questions are getting into the PSLE as well.

Hi ksi

From your earlier details,it appears that you are calculating the sum of the first 100 odd numbers from
1+3+5+7+9 + ………………………………………………………..191,193,195,197,199

Am I right to say that?

Best wishes

[corneyAmber]

tianzhu:

ksi:

Actually it is not tedious to work out, there is a pattern to calculate everything quite fast, I present more details for you to see but in exam, this can be done in 4 lines.


Having said this, this is more MO type of questions. However, I notice selective MO type of questions are getting into the PSLE as well.

Hi ksi

From your earlier details,it appears that you are calculating the sum of the first 100 odd numbers from
1+3+5+7+9 + ………………………………………………………..191,193,195,197,199

Am I right to say that?

Best wishes

Hi Tianzhu,

1+3+5+7+9 = 25 is common for all the first 100 odd numbers.
So I calculated this to be the constant 25 for each series of number with a different 1-digit and 2-digits starting.

1 digit starting...the adding pattern is 1*5, 2*5, 3*5 etc....9*5

2-digit starting....the adding pattern is 1*5+0*5, 1*5+1*5, 1*5+2*5,..... and this pattern can be further simplied.

So the constant is to be added for each series.