## Q&A - P5 Math

### Re: Q&A - P5 Math - Average with Unknown Quantlity

thanks for clarification speedmaths.com! I kind of just left it open typing the bracket and the words in it.speedmaths.com wrote:ILLUSDUCA wrote:Hello Jeff!JeffLim wrote:Hi there,

Need help on this.

The average mass of a group of boys is 45 kg.Ifone more boy who weighs 75 kg joins the group, the new average mass of the group will become 48 kg. Find the total number of boys in the group.

Thank you in advance.

Jeff

Here's one way to do this:

75kg - 45kg = 30kg difference

48kg - 45kg = 3kg difference

30/3 = 10 students (9 of them weighing average of 45kg and 1 of them weighing 75kg)

Hope this helps!

Hi,

The answer sh be9 boys,

as the boy who weighs 75 kg didNOTactually join the group.

Cheers

speedmaths

.

### Re: Q&A - P5 Math

Hello Sruthi,Sruthi wrote: _ Please help with this question

In figure 2, shaded areas are 6 small triangles.

So, 6 small triangles = 108cm^2

1 small triangle = 108 / 6 = 18 cm^2

And putting any 2 small triangles together forms a square, so, 18 x 2 = 36cm^2.

So, the area of any square formed by 2 small triangles = 36cm^2

Given that sides of any square will have the same length, and area of square = L x B, 36 = 6 x 6. The length of 1 side of the square is 6cm.

Figure 1 is made up of 16 (not 24, miscalculation) sides of a square, so it's perimeter is 16 x 6 = 96cm.

(thanks mathizzzfun for correction)

hope this helps!

Last edited by ILLUSDUCA on Wed Jul 16, 2014 11:23 am, edited 1 time in total.

- MathIzzzFun
- KiasuGrandMaster
**Posts:**2358**Joined:**Wed Mar 09,**Total Likes:**5

### Re: Q&A - P5 Math

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Last edited by MathIzzzFun on Wed Jul 23, 2014 3:52 pm, edited 1 time in total.

### Re: Q&A - P5 Math

Hi, I have doubts on the answer for this question:

A machine takes 60 seconds to cut an iron rod into 6 equal parts, At this rate, how long will it take the machine to cut the rod into 9 equal parts?

The book answer is 96s. Is it correct? How? I keep getting 90s.. Please advise. Thank you

A machine takes 60 seconds to cut an iron rod into 6 equal parts, At this rate, how long will it take the machine to cut the rod into 9 equal parts?

The book answer is 96s. Is it correct? How? I keep getting 90s.. Please advise. Thank you

- speedmaths.com
- BrownBelt
**Posts:**559**Joined:**Mon Oct 12,**Total Likes:**3

### Re: Q&A - P5 Math

Hi,tracywham wrote:Hi, I have doubts on the answer for this question:

A machine takes 60 seconds to cut an iron rod into 6 equal parts, At this rate, how long will it take the machine to cut the rod into 9 equal parts?

The book answer is 96s. Is it correct? How? I keep getting 90s.. Please advise. Thank you

"6 equal parts" means 5 cuts

"9 equal parts" means 8 cuts

5 cuts → 60 secs

1 cut → ??

8 cuts → ??

Can you solve from here?

Hope this helps.

**speedmaths**

.

- gingerbelle
- KiasuNewbie
**Posts:**2**Joined:**Thu Aug 14,

### Re: Q&A - P5 Math

Hi gingerbelle,gingerbelle wrote: Please help me with these 2 questions. Thank you.

1) A group of pupils calculated their average score for a Maths test. If one of them scored 9 marks more, their average score would be 81. If one of them scored 3 marks less, their average score would be 78. How many pupils were there in the group?

Not sure if it helps if you look at this problem as an “Excess/Shortage” scenario:

If everyone is to be given the score of 81, there is a shortage of 9 marks.

If everyone is only given the score of 78, there is an excess of 3 marks.

Excess/Shortage = 3 – (–9) = 3 + 9 = 12

Gap = 81 – 78 = 3

Number of pupils = 12/3 =

**4**

2) A group of 100 pupils participated in a Maths competition. Their average score was 76 marks. If the average score for the boys was 80 marks and for the girls was 70 marks, how many girls participated in the Maths competition?

Each boy contributed above the average: 80 – 76 = 4 marks

Each girl required 76 – 70 = 6 marks to meet the average.

The boys' marks above the average will compensate for the shortfall of the girls' marks:

1 girl (6 marks) would need 1.5 boy (4 marks x 1.5)

Therefore, 3 boys (3 x 4 = 12 marks) would help to bring up the score of 2 girls (2 x 6 = 12 marks) to the average.

So, there were 2 girls for every 3 boys => girls made up of 2/5 of the group.

Number of girls = 2/5 x 100 =

**40**

Hope the above is clear. Please feel free to clarify.

cheers...

- gingerbelle
- KiasuNewbie
**Posts:**2**Joined:**Thu Aug 14,

### Re: Q&A - P5 Math

Thank you so much for your clear explanation. I got the answers through trial and error but could not explain it to my son. That's why he wrote "dunno why" on his worksheet

- speedmaths.com
- BrownBelt
**Posts:**559**Joined:**Mon Oct 12,**Total Likes:**3

### Re: Q&A - P5 Math

Hi,tracywham wrote:Hi, I have doubts on the answer for this question:

A machine takes 60 seconds to cut an iron rod into 6 equal parts, At this rate, how long will it take the machine to cut the rod into 9 equal parts?

The book answer is 96s. Is it correct? How? I keep getting 90s.. Please advise. Thank you

"6 equal parts" means 5 cuts

"9 equal parts" means 8 cuts

5 cuts → 60 s

1 cut → 12 s

8 cuts →

**96 s**

Hope this helps.

**speedmaths**

.