Q&A - PSLE Math

[tianzhu]

kavisuresh:

1. Study the five layer block as shown below. It is built from 55 similar 1cm cubes and is painted green. Find the total surface area that is painted green.
http://i54.tinypic.com/s6nhtt.jpg\">

Hi

Where is this question from?

We cannot read the question in full.

Assuming to be a portable toy structure not a fixed structure.This means the bottom of the structure is also painted.

When viewed from the front or the back, the left or the right, we can see (1+2+3+4+5) or 15 square faces.

When viewed from the top or bottom, we can see 25 square faces.

Number of square faces ------- 4*15 + 2*25 ------ 110

Surface area to be painted ------110*1*1 ------ 110 sq cm

Best wishes

[tianzhu]

htn:

tianzhu
Thank you for your help.

Hi

You’re welcome.

Best wishes

[Angel25]

Pls HELP!!



Roy jogged from his house to the playground and back at 10km/h. He reached home at 12.18pm. If he jogged at 3km/h slower, he would reach home at 1.30pm. If he wanted to reach home at 12.45pm to have his lunch, at what speed did he have to jog?

Thanks!!

[Daddy]

Thanks MathIsssFun,


For Q2 i did not understand. Please explain. Thanks
http://i53.tinypic.com/10hlv08.jpg\">

How did you come out the model for Sam?
Why $40 = to 10u?
Why 7u = 2u + 10 +80?

Thank.

[maths6a]

Hi need help with this qn.

Has anyone posted it yet? pl direct me to the page. Thanks.

A, B and C were standing in a straight line waiting for a race to start.
C was 300 m ahead of B and B was 100 m ahead of A. At 9 am, they started the race. A overtook B in 5 min. In another 5 min, A overtook C. If B's speed is 150 m/min, at what time did B overtake C?

Thanks in advance.

[MathIzzzFun]

Daddy:

Thanks MathIsssFun,


For Q2 i did not understand. Please explain. Thanks
http://i53.tinypic.com/10hlv08.jpg\">

How did you come out the model for Sam?
Why $40 = to 10u?
Why 7u = 2u + 10 +80?

Thank.

Hi

hope this explains..

http://i52.tinypic.com/4qfu2p.jpg\">

cheers.

[smileplease]

Anyone knows how to do this Q :


There were 1960 workers in a garment factory this year. This is 840 less than that of last year. Find the percentage decrease in the number of workers?

How do we determine it is 840/2800 x 100 OR 840/1960 x 100?

thanks.

[MathIzzzFun]

maths6a:

Hi need help with this qn.

Has anyone posted it yet? pl direct me to the page. Thanks.

A, B and C were standing in a straight line waiting for a race to start.
C was 300 m ahead of B and B was 100 m ahead of A. At 9 am, they started the race. A overtook B in 5 min. In another 5 min, A overtook C. If B's speed is 150 m/min, at what time did B overtake C?

Thanks in advance.

Hi

A made up 100m to catch up with B in 5 min.
100m/5 min = 20 m/min, so A is 20 m/min faster than B, ie A's speed = 170 m/min

A made up 400 m to catch up with C in 10 min.
400m/10min = 40m/min, A's is 40m/min faster than C.
C's speed = 170m/min - 40m/min = 130m/min

150m/min - 130/min = 20 m/min - B is 20 m/min faster than C
300m / 20m/min = 15 min
B will overtake C 15 min later, at 9.15am

cheers.

[maths6a]

Correct me if I am wrong, but

840/2800 x 100% is the correct answer because percentage decrease is compared to what was at first/original which in this case was last year.

[atutor2001]

smileplease:

Anyone knows how to do this Q :


There were 1960 workers in a garment factory this year. This is 840 less than that of last year. Find the percentage decrease in the number of workers?

How do we determine it is 840/2800 x 100 OR 840/1960 x 100?

thanks.

This one I know.
Decrease is changing from BIG to small so the original/start is BIG -> divide by BIG
Increase is changing from SMALL to big so the original/start is SMALL -> divide by SMALL
That was how I told my kids in those good old days.