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Probability question

2022 · 29 Jun · Shift 1 · Q20
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Probability question

2022 · 29 Jun · Shift 1 · Q20

JEE MainMathematicsProbabilityMCQ+4 / −1
The probability that a randomly chosen 2 ×\times× 2 matrix with all the entries from the set of first 10 primes, is singular, is equal to :
  1. A
    133104{{133} \over {{{10}^4}}}104133​
  2. B
    18103{{18} \over {{{10}^3}}}10318​
  3. C
    19103{{19} \over {{{10}^3}}}10319​
  4. D
    271104{{271} \over {{{10}^4}}}104271​
View written solutionFree

Correct answer: C

Let the matrix be A=(abcd)A=\begin{pmatrix} a & b \\ c & d \end{pmatrix}A=(ac​bd​) where each entry is chosen from the set of first 101010 primes: S={2,3,5,7,11,13,17,19,23,29}S=\{2,3,5,7,11,13,17,19,23,29\}S={2,3,5,7,11,13,17,19,23,29}

Since each of the 444 entries can be chosen independently in 101010 ways, the total number of matrices is 104=10000.10^4=10000.104=10000.

We need the probability that the matrix is singular.

1. Condition for singularity

A 2×22\times 22×2 matrix is singular iff its determinant is zero: ad−bc=0ad-bc=0ad−bc=0 that is, ad=bc.ad=bc.ad=bc.

So we must count the number of ordered quadruples (a,b,c,d)∈S4(a,b,c,d)\in S^4(a,b,c,d)∈S4 such that ad=bc.ad=bc.ad=bc.


2. Use prime factorization structure

All entries are prime numbers.

If ad=bc,ad=bc,ad=bc, then the product of the two primes on the left equals the product of the two primes on the right.

By unique factorization, the multiset of primes in (a,d)(a,d)(a,d) must be the same as the multiset of primes in (b,c)(b,c)(b,c).

So the only possibilities are:

  1. a=ba=ba=b and d=cd=cd=c, or
  2. a=ca=ca=c and d=bd=bd=b.

Equivalently, once (a,d)(a,d)(a,d) is chosen, (b,c)(b,c)(b,c) must be either (a,d)(a,d)(a,d) or (d,a)(d,a)(d,a).


3. Count singular matrices carefully

We count ordered pairs (a,d)(a,d)(a,d) first.

There are 10×10=10010\times 10=10010×10=100 choices for (a,d)(a,d)(a,d).

Case 1: a≠da\neq da=d

Number of such pairs: 10⋅9=90.10\cdot 9=90.10⋅9=90.

For each such pair, (b,c)(b,c)(b,c) can be chosen in exactly 222 ways: (b,c)=(a,d)or(d,a).(b,c)=(a,d) \quad \text{or} \quad (d,a).(b,c)=(a,d)or(d,a). So contribution is 90⋅2=180.90\cdot 2=180.90⋅2=180.

Case 2: a=da=da=d

Number of such pairs: 10.10.10.

Then the two possibilities for (b,c)(b,c)(b,c) coincide, since (a,d)=(d,a).(a,d)=(d,a).(a,d)=(d,a). So for each such pair there is only 111 valid choice: (b,c)=(a,a).(b,c)=(a,a).(b,c)=(a,a). Thus contribution is 10⋅1=10.10\cdot 1=10.10⋅1=10.

Hence total number of singular matrices is 180+10=190.180+10=190.180+10=190.


4. Compute probability

Therefore, P(singular)=190104=191000.P(\text{singular})=\frac{190}{10^4}=\frac{19}{1000}.P(singular)=104190​=100019​.


5. Compare with options

191000\frac{19}{1000}100019​ corresponds to Option C.


Final Answer

19103\boxed{\frac{19}{10^3}}10319​​

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