Q&A - PSLE Math

[tianzhu]

Hi zsqchx


This is a two scenarios question on \"double if \". It's quite an interesting one.

Hope this helps.

Best wishes

http://farm6.static.flickr.com/5066/5590853745_0e5826520a_z.jpg\">

[jieheng]

zsqchx:

Hi, Someone can help for the following question. Thanks!


There are some red and blue pencils in a box. If 30 red pencils are taken out from the box. the total number of pencils left will be 8 times the number of red pencils left. If 50 blue pencils are taken out from the box. the total number of pencils left will be thrice that of the number of blue pencils left. How many pencils are there in the box?

Model 1
R [ ][30]

B [ ][ ][ ][ ][ ][ ][ ]

When 50 blue pencils are taken out , ration of red pencils to the blue pencils is 2 : 1

Modified the model 1 , [ ] = [ ][ ]
Model 1
R [ ][15][ ][15]

B [ ][ ][ ][ ][ ][ ][ ][ ][ ][ ][ ][ ][ ][ ] (14u)

Model 2
R [ ][15][ ][15] (2u+30)

B [ ][15][----------------50---------------] (1u+15+50)

Compare the Blue pencils of the models 1 and 2

14u = 1u + 15 + 50

13u = 65

1u = 5

From the model 1 ,

Total number of the pencils in the box = 16u + 30 = 16 * 5 + 30 = 110 (Ans)

[zsqchx]

Thank you! Tianzhu and Jie Heng

[tianzhu]

zsqchx:

Thank you! Tianzhu and Jie Heng

Hi

You're welcome.

Best wishes.

[tianzhu]

jesschan:

This is a P5 maths question - not sure is this the correct place to post.


A man spent $1119 on a diamond using $50, $5 and $2 notes. The total number of notes used is 45. How many $50, $5 and $2 notes did he use?

I know how to solve it using Guess and Check - just wondering if there is a quicker method?

Hi jesschan

I did a quick check, is the answer, 20 pieces of $50, 23 pieces of $5 and 2 pieces of $2?

I’ll try to consolidate my thoughts at a later time to present the solution in more details if the answer is correct.

Best wishes

[jieheng]

zsqchx:

Thank you! Tianzhu and Jie Heng

You are welcome.

Regards,

[shurley197323]

Pls help.

In a school hall, a certai no of students can be arranged such that there are exactly 21 students per row. If the same no of students is arranged in rows of 16 students each. there will be 7 more rows and 8 students left over. How many students are there altogether?
Answer given is 504.
Thanks

[BeContented]

Let R be the number of rows.


21R = 16 (R+7) + 8
= 16R + 120
5R = 120
R = 24

So 21 * 24 = 504

Correct right?!?!

[jesschan]

tianzhu:

jesschan:

This is a P5 maths question - not sure is this the correct place to post.


A man spent $1119 on a diamond using $50, $5 and $2 notes. The total number of notes used is 45. How many $50, $5 and $2 notes did he use?

I know how to solve it using Guess and Check - just wondering if there is a quicker method?

Hi jesschan

I did a quick check, is the answer, 20 pieces of $50, 23 pieces of $5 and 2 pieces of $2?

I’ll try to consolidate my thoughts at a later time to present the solution in more details if the answer is correct.


Yes, it's correct.

Best wishes

[shurley197323]

cwc:

Let R be the number of rows.


21R = 16 (R+7) + 8
= 16R + 120
5R = 120
R = 24

So 21 * 24 = 504

Correct right?!?!

Hi. Thanks cwc. But why 5R = 120?
Why must 120 / 5 ?
How to explain? Thanks