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Seven (distinct) car accidents occurred in a week. What is the probability that they all occurred on the same day?

  1. $\dfrac{1}{7^7}\\$
  2. $\dfrac{1}{7^6}\\$
  3. $\dfrac{1}{2^7}\\$
  4. $\dfrac{7}{2^7}\\$
asked in Probability by Veteran (59.5k points)
edited by | 2.9k views
0
@chotu can u describe it an easy way?

2 Answers

+20 votes
Best answer

Answer - B 

for every car accident we can pick a day in $7$ ways

total number of ways in which accidents can be assigned to days $= 7^{7}$

probability of accidents happening on a particular day $=\dfrac{1}{7^{7}}$

we can choose a day in $7$ ways.

hence probability $=\dfrac{7}{7^{7}}=\dfrac{1}{7^{6}}.$

answered by Loyal (9k points)
edited by
+8

the answer is B. The number of ways you can choose the "same day" is 7. The probability of all the accidents happening on same day is 1/77 . So 7*(1/77) is 1/76.

0
i missed that i'll edit the answer
+13 votes

P(accident on a single day out of 7 days)=$\frac{1}{7}$

P(all accident occurred on the same day Out of 7 Day)= $\binom{7}{1}$ $(\frac{1}{7})^{7}$ $(\frac{6}{7})^{0}$   = $\frac{7}{7^{7}}$ = $\frac{1}{7^{6}}$  //Binomial Distribution So Option B is Correct Ans

(Means selecting single day out of 7 days and all the 7 accident should happen on same day with probability $\frac{1}{7}$)

answered by Boss (22.7k points)
edited by
0


@Rajesh Pradhan @ankitrokdeonsns @Arjun
Just wanted to know whether we can calculate Sample space by using 'Combinations with repetition' ( sum of non negative integral solutions)concept which says

If x1 + x2+......xn= r( xi >=0) then no. of combination possible is C(n-1+r, r).

If so, then in above problem

x1+ x2 +x3+x4+x5+x6+x7= 7

which gives us sample space= C(7-1+7, 7) Therefore required probability is 7C1/ 13C7

If this logic cant be applied please explain why?

Thank you

0
same doubt here.please someone explain this.
0
I understood why my above mentioned approach is wrong.

First of understand the solution.

7 accidents occurred in a week say a1, a2,....a7.

No of different ways accident can occur is all accidents (a1......a7) have 7choices. Therefore required probability = $\dfrac{7}{7^7}\\$

The approach mentioned in my above comment is wrong because,each accident is unique(a1....a7) which can occur in any day of week.

So if apply x1+x2 ..+x7 = 7 here, 1 combination will be x1= 5, x2= 2 amd rest all equal to 0. But x1 = 5 have 7C5 combination as all accidents are unique and any 5 out of 7 accidents can go in x1. So using this approach is wrong for this problem.


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