Tutor MathsGuru: Ask me for your burning Maths questions!

[Emelyn]

mathsguru:

Hi Emelyn,


Please take a look at my solution. A quick & dirty way (but surely correct way) is to use what accountants would term as debit and credit.

Of course, this may be too difficult for lower primary students to understand. You can explain it as money forked out vs. money received. The difference will be the gain/loss depending on which amount is bigger.

Hope this helps!


MathsGuru

http://www.postimage.org/image.php?v=Pq8_Pv0

Thanks Mathsguru and EVERYONE who responded.

Like CoffeeCat, we (5 of us in the office working on the qn) were very disturbed with :

900 - 850 = 50.

which is also what the 2 school teachers of my colliq's dd1 and dd2 did.

For a moment, we were wondering is this a new way of calculating, is this a trick qn, and why we cannot solve a P3 question !!!!

Phew.... thanks for giving our self-confidence back....

[firebird]

Good morning Mathsguru


Please help me on the following P6 maths:

1) Clement had a wooden block measuring 50cm by 70cm by 90cm.
He wanted to cut out some little cuboids measuring 2cm by 3cm by 4cm.
What is the minimum amount of wood wasted?

Given answer is 360cm cuboids. I am getting 180 cuboids. Please help.

2) Micheal asked his classmate what time it was.
Jackson, the math whiz, replied with a cheeky grin,
"It’s 2000 hours, 1990 minutes and 1980 seconds before midnight?
Can you help Micheal find out what time it is?

I am totally lost.

Thank you
firebird

[firebird]

Good morning Dharma


Many thanks
firebird

[CoffeeCat]

firebird:

Good morning Mathsguru


Please help me on the following P6 maths:

1) Clement had a wooden block measuring 50cm by 70cm by 90cm.
He wanted to cut out some little cuboids measuring 2cm by 3cm by 4cm.
What is the minimum amount of wood wasted?

Given answer is 360cm cuboids. I am getting 180 cuboids. Please help.

2) Micheal asked his classmate what time it was.
Jackson, the math whiz, replied with a cheeky grin,
\"It's 2000 hours, 1990 minutes and 1980 seconds before midnight?
Can you help Micheal find out what time it is?

I am totally lost.

Thank you
firebird

edited : my first try wrong look at newer post.

[Dharma]

firebird:

2) Micheal asked his classmate what time it was.
Jackson, the math whiz, replied with a cheeky grin,
\"It's 2000 hours, 1990 minutes and 1980 seconds before midnight?
Can you help Micheal find out what time it is?

I am totally lost.

Thank you
firebird

2000 hrs + 1990 mins + 1980 sec
= 2000 + (1990 x 60 + 1980)/3600
= (2000 + 33 + 43/60) hrs
= (1992 + 8 + 24 + 9 + 43/60)hrs
= 84 days + 17hrs + 43/60 hrs

To find out time …. 17 hrs and 43 mins before midnight i.e. 6.17am

[firebird]

Dear coffeeCat & Dharma


Thanks for your reply.

The question one answer given by coffeecat is not as the same as the given answer - i.e. 360 cm3. I really do not know!

The question two answer given by Dharma is same as given answer - i.e 6.17am. However, I could not understand the working.

Regards
firebird

[CoffeeCat]

oh yea i underestimated the question.

i think i know how the setter got his 360.

Due to the complexity of this question we shall break it up into 2 parts, the initial cutting and the subsequent cuttings

first notice that only 3 divides 90 evenly, and 50/3 = 16 r2 , 70/3=23 r1,
2 divides 50 and 70 and 90 evenly while 90, 70 & 50 leave a remainder of 2 upon division by 4.

initial cutting
if we divide 70 by 3, we will be left with an unuseable wastage of 1*50*90, too large so we will rule this out.

For the rest of the configurations, we will be left with wastages with 1 side of 2cm long, which will be dealt with in the subsequent cutting part.

Intuitively this might be the most efficient because wastage is confined to only 1 side of the block.
Let's say we only want to create wastage along 1 side, the only 2 configurations are (3 will have to divides into 90)
a) (90/3) & (70/2) & (50/4)
The initial cutting yields (30 * 35 * 12)=12600 cuboids
and we are left with a 2*70*90 board.
This will yield either 506 or 510 additional cuboids.
Total = 13106 or 13100 cuboids, leaving 600 cubic cm wastage for 13100 cuboids..

b) (90/3) & (50/2) & (70/4)
The initial cutting yields (30*25*17)=12750 cuboids
and left with a 2*50*90 board.
which will yield either 352 or 360 additional cuboids.
therefore wastage is 360 cubic cm for 13110 cuboids (this is the configuration the setter is looking for)

Lets say we end up creating wastage along 2 sides after the initial cutting.
then there are only 2 more cases to consider (when 3 divides into 50)
a) (50/3) & (90/4) & (70/2)
b) (50/3) & (70/4) & (90/2)


Maybe you can share how you got your 180 wastage.

[Dharma]

firebird:

Dear coffeeCat & Dharma


Thanks for your reply.

The question one answer given by coffeecat is not as the same as the given answer - i.e. 360 cm3. I really do not know!

The question two answer given by Dharma is same as given answer - i.e 6.17am. However, I could not understand the working.

Regards
firebird

Hi firebird,

Q2
2000 hrs + 1990 mins + 1980 sec
= 2000 + (1990 x 60 + 1980)/3600
= (2000 + 33 + 43/60) hrs
= (1992 + 8 + 24 + 9 + 43/60)hrs
= 84 days + 17hrs + 43/60 hrs

84 days before midnight is also midnight. So, we left with 17 hrs and 43 mins before midnight.

Time is 17 hrs and 43 mins before midnight i.e. 6.17am

[OngMum]

Thank you, Tang for the link on the correct qns phrasing.

Thank you, Mathsguru for the problems solving.

Have a good day!

Cheers!

[YLH88]

Hi Mathsguru,


Thank you for the solution!

Regards