Dencology

Full Member
10+ Year Member
Mar 7, 2008
396
0
Status (Visible)
  1. Pre-Dental
what is the probability of throwing two fours with a dice in five tries?

ans.
10(125)/36(216)

how?
 

Dentista08

looking for a practice!
10+ Year Member
5+ Year Member
Jul 25, 2008
228
0
Boston, MA
Status (Visible)
  1. Dental Student
  2. Dentist
what is the probability of throwing two fours with a dice in five tries?

ans.
10(125)/36(216)

how?
i HATE these problems!!!
So they use this stupid formula where you have to figure out the probability of getting a 4 when you throw a dice. It is 1/6, right? Bc theres 1 4 in a 6 faced die.
Now the probability of NOT getting a 4 in 1 throw is (5/6) (five other numbers- 1,2,3,5,6 in a die)
So we have a formula Prob of r success out of n attempts=
nr(p^r)(q^(n-r))


n=5 attemps
r= success out of 5 attempts= 2
p=prob of success in 1 throw= 1/6
q= prob. of failure in 1 throw= 1-p= 1-(1/6) = 5/6
Since we have to throw a four 2 times, we bring (1/6)^2
Since we have failure the rest of the times (5times total- 2successful times= 3 times of failure) we bring up the failure to the power of 3 (5/6)^3
So, we get
5(2)(1/6^2)(5/6^(5-2))=
(10) (1/36) (125/216)= (10)(1)(125)/ (36)(216)
 

Streetwolf

Ultra Senior Member
10+ Year Member
7+ Year Member
Oct 25, 2006
1,801
7
NJ
Status (Visible)
  1. Dentist
what is the probability of throwing two fours with a dice in five tries?

ans.
10(125)/36(216)

how?

Choose the 2 rolls that are 4s = (5 C 2), account for the 2 successes (1/6)^2 and account for the 3 failures (5/6)^3.

(5 C 2) = 10.

You get 10(125)/(36)(216)

The 10 is from (5 C 2), the 125 is 5^3, the 36 is 6^2, and the 216 is 6^3.
 

jay47

Think Positively!
10+ Year Member
5+ Year Member
Aug 4, 2007
1,068
8
Status (Visible)
  1. Dentist
Choose the 2 rolls that are 4s = (5 C 2), account for the 2 successes (1/6)^2 and account for the 3 failures (5/6)^3.

(5 C 2) = 10.

You get 10(125)/(36)(216)

The 10 is from (5 C 2), the 125 is 5^3, the 36 is 6^2, and the 216 is 6^3.

Amazing, abolutely amazing. I swear I have never seen a math question that Streetwolf did NOT answer in the year and a half I have been on SDN.
 
About the Ads

Streetwolf

Ultra Senior Member
10+ Year Member
7+ Year Member
Oct 25, 2006
1,801
7
NJ
Status (Visible)
  1. Dentist
They are independent events so it won't matter. The thing is you'd have to roll 2 dice two times (4 rolls total) and then take one die and roll it once for the 5th roll.
 

Dencology

Full Member
10+ Year Member
Mar 7, 2008
396
0
Status (Visible)
  1. Pre-Dental
i HATE these problems!!!
So they use this stupid formula where you have to figure out the probability of getting a 4 when you throw a dice. It is 1/6, right? Bc theres 1 4 in a 6 faced die.
Now the probability of NOT getting a 4 in 1 throw is (5/6) (five other numbers- 1,2,3,5,6 in a die)
So we have a formula Prob of r success out of n attempts=
nr(p^r)(q^(n-r))


n=5 attemps
r= success out of 5 attempts= 2
p=prob of success in 1 throw= 1/6
q= prob. of failure in 1 throw= 1-p= 1-(1/6) = 5/6
Since we have to throw a four 2 times, we bring (1/6)^2
Since we have failure the rest of the times (5times total- 2successful times= 3 times of failure) we bring up the failure to the power of 3 (5/6)^3
So, we get
5(2)(1/6^2)(5/6^(5-2))=
(10) (1/36) (125/216)= (10)(1)(125)/ (36)(216)


If there is a 70% chance of rain each day for the next 3 days. what is the probability it will rain 2 out of the 3 days?

so i did what you said:

3(2)(7/10)^2(3/10)^1

which it comes down to

6(49/100)(3/10)

but the answer is
3(49/100)(3/10). how?
 

klutzy1987

StudyingSucks Letsgo Mets
10+ Year Member
5+ Year Member
Jan 28, 2008
2,134
3
New York
Status (Visible)
  1. Dental Student
If there is a 70% chance of rain each day for the next 3 days. what is the probability it will rain 2 out of the 3 days?

so i did what you said:

3(2)(7/10)^2(3/10)^1

which it comes down to

6(49/100)(3/10)

but the answer is
3(49/100)(3/10). how?

3c2 * .7^2 * .3^1

This equals 3*.49 * .3 which is the correct answer. You calculated 3c2 wrong.
 

klutzy1987

StudyingSucks Letsgo Mets
10+ Year Member
5+ Year Member
Jan 28, 2008
2,134
3
New York
Status (Visible)
  1. Dental Student
Heres a question for streetwolf that he will find quite challenging.

if you have a branch that is 10 inches in diameter and there is a rope that spirals around it. The vertical distance between each complete revolution is 35 inches. How much rope is required to make on complete revolution.
(remember that the rope is spiralling up the branch so you need to account for the thickness of the branch as well is the vertical height of the revolution). I think you may enjoy this one streetwolf.
 

Dencology

Full Member
10+ Year Member
Mar 7, 2008
396
0
Status (Visible)
  1. Pre-Dental
3c2 * .7^2 * .3^1

This equals 3*.49 * .3 which is the correct answer. You calculated 3c2 wrong.


how did you get 3*.49 * .3. i don't get this three. i thought i would be 6 because of this formula: nr(p^r)(q^(n-r))

n=3 attemps
r= success out of 3 attempts= 2
p=prob of success in 1 throw= 70%
q= prob. of failure in 1 throw= 1-p= 1-(0.7) = 0.3

i followed this and still the answer would come out wrong. Why?
 

Streetwolf

Ultra Senior Member
10+ Year Member
7+ Year Member
Oct 25, 2006
1,801
7
NJ
Status (Visible)
  1. Dentist
Heres a question for streetwolf that he will find quite challenging.

if you have a branch that is 10 inches in diameter and there is a rope that spirals around it. The vertical distance between each complete revolution is 35 inches. How much rope is required to make on complete revolution.
(remember that the rope is spiralling up the branch so you need to account for the thickness of the branch as well is the vertical height of the revolution). I think you may enjoy this one streetwolf.
If you unravel the outer coating of the branch with the rope still attached, within the area of exactly 1 revolution, what you'd get is a rectangular sheet of the branch with height (vertical) = 35 inches and length (horizontal) = 10pi inches (this is the circumference of the branch). The rope itself would run on the diagonal from bottom left to top right (or bottom right to top left, whatever you choose).

Either way, the length of the rope is found with a^2 + b^2 = c^2. Here we have a = the height and b = the width. You get c = 47.03 inches.
 

Dentista08

looking for a practice!
10+ Year Member
5+ Year Member
Jul 25, 2008
228
0
Boston, MA
Status (Visible)
  1. Dental Student
  2. Dentist
how did you get 3*.49 * .3. i don't get this three. i thought i would be 6 because of this formula: nr(p^r)(q^(n-r))

n=3 attemps
r= success out of 3 attempts= 2
p=prob of success in 1 throw= 70%
q= prob. of failure in 1 throw= 1-p= 1-(0.7) = 0.3

i followed this and still the answer would come out wrong. Why?
to be honest, i dont get it either, why not 3 & not 6 * .49 * .3??
 
This thread is more than 12 years old.

Your message may be considered spam for the following reasons:

  1. Your new thread title is very short, and likely is unhelpful.
  2. Your reply is very short and likely does not add anything to the thread.
  3. Your reply is very long and likely does not add anything to the thread.
  4. It is very likely that it does not need any further discussion and thus bumping it serves no purpose.
  5. Your message is mostly quotes or spoilers.
  6. Your reply has occurred very quickly after a previous reply and likely does not add anything to the thread.
  7. This thread is locked.