Q&A - PSLE Math

[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.

[Brenda10]

[quote=\"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?

Suggest use guess and check:

Camera - $60, Watch - $15
Camera is 40% and watch is 60% of total purchase

Camera Watch Total

4 x $60 6 x $15 = $330
12 x $60 18 x $15 = $990
24 x $60 36 x $15 = $1980 (Correct Answer)


Mr Mohn bought 36 watches.

Hope the answer is correct.
:lol:

[tianzhu]

ksi:

tianzhu:

[quote=\"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.[/quote]Hi ksi

If you are calculating the sum of the first 100 odd numbers, the answer is unlikely to be 1000.

Take for example, the sum of the last few first 100 odd numbers

181+183+185+187+189+191+193+195+197+199

They almost add up to 1000 already.

Best wishes

[liketoeat]

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?

C : W
4 : 6
= 2 : 3

Cost of W = $60/4 = $15
Cost of 1 set of W + C = ( 3 x $15) + (2 x $60) = $165

No. of sets = $1980/$165 = 12

No. of W = 12 x 3 = 36

[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

Where is your question from?
Is there any way to verify the answer?

Best wishes

[liketoeat]

tianzhu:

ksi:

[quote=\"tianzhu\"]
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.

Hi ksi

If you are calculating the sum of the first 100 odd numbers, the answer is unlikely to be 1000.

Take for example, the sum of the last few first 100 odd numbers

181+183+185+187+189+191+193+195+197+199

They almost add up to 1000 already.

Best wishes[/quote]
You are not to add the numbers but the digit....i.e. 1 + 8 + 1 + 1 + 8 + 3 .........

[liketoeat]

tianzhu:

Hi

Where is your question from?
Is there any way to verify the answer?

Best wishes

Sorry, I don't know where the questions came from, it was given to me by my niece from her school teacher.

[tianzhu]

liketoeat:

You are not to add the numbers but the digit....i.e. 1 + 8 + 1 + 1 + 8 + 3 .........

Hi

Thank you

Best wishes