whats the answer?? (1 Viewer)

[Dumbarse]

from a pack of nine cards numbered 1,2,3,......,9 , three cards are drawn at random and laid on a table from left to right

1) what is the probability that the number formed exceeds 400?

11) what is the probability that the digits are drawn in descending order??

 

[Will]

nb: we're dealing with ordered arrangements

i)

you can have 4xx 5xx 6xx... 9xx
ie 6 different 1st digits
then 8 * 7 ways of choosing the last 2 digits
ie 8P3

total combinations = 9P3

thus, P = 8P3/9P3 = 2/3

ii)

do each case selectively
for first digit 9, you have 8 possibilities for 2nd digit and 7 possiblities for 3rd = 1 * 8 * 7
for first digit 8, you have 7 possibilities for 2nd and 6 for 3rd = 1 * 7 * 6
and you do that until first digit is 3 (can't be 2 or 1 obviously)

so total combinations = 8*7 + 7*6 + 6*5 + 5*4 + 4*3 + 3*2 + 2*1

P = that/9P3 = 1/3

 

[gumpy]

for the second part i got 1/6

i did 7 + 2*6+3*5+4*4+5*3+6*2+7*1= whatever

if it's wrong oh well

 

[BlackJack]

For part 2, Think of it this way:

I'lll critique the answer...

Let's pick the 8 as the first digit,
but the second digit can't be 1, therefore there's six possibilities.
For eachof these six, the possibiities is one less than the number
hence for the 8, it is 1 * (1+2+3+4+5+6)
which is.. 7C2
The sum of ways is from 2c2 (case of 3) to 8c2 (case of 9)
which is 84.
Divide by 9P3.
answer. 1/6

 

[spice girl]

Here's a nice elegant way for part (ii)

Select any combination of three cards.

Of arranging these three cards, there are six ways of doing so, but only one of these will give you decreasing order.

Hence P = 1/6

:jaw:

 

[BlackJack]

Ah, thankyou. I keep on forgetting shortcuts shortly after I discover it.
Tried to remember it then... but couldn't, hence long answer.

 

[McLake]

Originally posted by spice girl
Here's a nice elegant way for part (ii)

Select any combination of three cards.

Of arranging these three cards, there are six ways of doing so, but only one of these will give you decreasing order.

Hence P = 1/6

:jaw:

Oooh, very nice. Annoyed with that question now.

So spice "smarty pance" girl, any question that you could NOT do ?

 

[macca202]

ahhh shit shit, i thought it ment 987 876 765 654 543 432 and 321. Damn that to hell!! I think the answer i have 4 this very simple 3 U q is wrong.

whats d/dx of tan(^-1)2x ?? is it 1/sqrt( 1 + 4 x^2) or 2/sqrt...etc??

ignoredis

 

[saladsurgery]

spicegirls right on the money!
for part i just think any number that starts with a number > 3 will be bigger than 400.
chances of first number >3 = 6/9 = 2/3

 

[BlackJack]

Originally posted by macca202

whats d/dx of tan(^-1)2x ?? is it 1/sqrt( 1 + 4 x^2) or 2/sqrt...etc??

2/sqrt( 1 + 4 x^2)

 

[josie_is_slut]

THIS IS IMPORTANT
DOES ANYONE KNOW HOW TO DIFFIRENTITATE X^2

 

[Ozz^E]

Originally posted by josie_is_slut
THIS IS IMPORTANT
DOES ANYONE KNOW HOW TO DIFFIRENTITATE X^2

is this a trcik question or sumfing...id say tha answer is 2X.