Sign in
12thPass logo
New chatPYQ LibraryDoubtsRank report
Sign in to see Recents

Your guest activity stays on this device

Sign in to save progress →
Sign in

Permutations and Combinations question

2023 · 31 Jan · Shift 2 · Q40
Guest · filters and generic practice availableBrowsing as a guest · PYQ filters and generic practice are available. Sign in only for personalised features and saved progress.
  1. PYQ Library
  2. /JEE Main
  3. /Mathematics
  4. /Permutations and Combinations
  5. /2023 · 31 Jan · Shift 2 · Q40

Permutations and Combinations question

2023 · 31 Jan · Shift 2 · Q40

JEE MainMathematicsPermutations and CombinationsNumerical+4 / −1
Let A=[aij],aij∈Z∩[0,4],1≤i,j≤2\mathrm{A}=\left[\mathrm{a}_{i j}\right], \mathrm{a}_{i j} \in \mathbb{Z} \cap[0,4], 1 \leq i, j \leq 2A=[aij​],aij​∈Z∩[0,4],1≤i,j≤2. The number of matrices A such that the sum of all entries is a prime number p∈(2,13)\mathrm{p} \in(2,13)p∈(2,13) is ‾\underline{\hspace{2cm}}​.
Numerical answer
View written solutionFree

Correct answer: 204

We need to count the number of 2×22\times 22×2 matrices A=[a11a12a21a22]A=\begin{bmatrix}a_{11}&a_{12}\\ a_{21}&a_{22}\end{bmatrix}A=[a11​a21​​a12​a22​​] with each entry in {0,1,2,3,4}\{0,1,2,3,4\}{0,1,2,3,4} such that the sum of all four entries is a prime number in (2,13)(2,13)(2,13).

1. Possible prime sums

Primes in the interval (2,13)(2,13)(2,13) are 3,5,7,11.3,5,7,11.3,5,7,11. So we must count the number of ordered quadruples (a11,a12,a21,a22),aij∈{0,1,2,3,4}(a_{11},a_{12},a_{21},a_{22}), \qquad a_{ij}\in\{0,1,2,3,4\}(a11​,a12​,a21​,a22​),aij​∈{0,1,2,3,4} with total sum equal to 3,5,7,3,5,7,3,5,7, or 111111.

Thus the problem reduces to finding the coefficient of x3+x5+x7+x11x^3+x^5+x^7+x^{11}x3+x5+x7+x11 in (1+x+x2+x3+x4)4.(1+x+x^2+x^3+x^4)^4.(1+x+x2+x3+x4)4.

We will count each sum separately.


2. Count solutions for sum 333

We need the number of nonnegative integer solutions of x1+x2+x3+x4=3,x_1+x_2+x_3+x_4=3,x1​+x2​+x3​+x4​=3, with each xi≤4x_i\le 4xi​≤4.

Since the sum is only 333, the upper bound ≤4\le 4≤4 is automatically satisfied.

Number of solutions: (3+4−14−1)=(63)=20.\binom{3+4-1}{4-1}=\binom{6}{3}=20.(4−13+4−1​)=(36​)=20.

So, N(3)=20.N(3)=20.N(3)=20.


3. Count solutions for sum 555

We need the number of solutions of x1+x2+x3+x4=5,0≤xi≤4.x_1+x_2+x_3+x_4=5, \qquad 0\le x_i\le 4.x1​+x2​+x3​+x4​=5,0≤xi​≤4.

First count all nonnegative solutions: (5+4−13)=(83)=56.\binom{5+4-1}{3}=\binom{8}{3}=56.(35+4−1​)=(38​)=56.

Now subtract those where some variable is at least 555.

If x1≥5x_1\ge 5x1​≥5, write x1′=x1−5≥0x_1'=x_1-5\ge 0x1′​=x1​−5≥0. Then x1′+x2+x3+x4=0,x_1'+x_2+x_3+x_4=0,x1′​+x2​+x3​+x4​=0, which has exactly 111 solution. Similarly for each of the 4 variables.

No two variables can both be at least 555 because the total sum is 555.

Hence N(5)=56−4=52.N(5)=56-4=52.N(5)=56−4=52.


4. Count solutions for sum 777

We need the number of solutions of x1+x2+x3+x4=7,0≤xi≤4.x_1+x_2+x_3+x_4=7, \qquad 0\le x_i\le 4.x1​+x2​+x3​+x4​=7,0≤xi​≤4.

Total nonnegative solutions: (7+4−13)=(103)=120.\binom{7+4-1}{3}=\binom{10}{3}=120.(37+4−1​)=(310​)=120.

Subtract solutions with some xi≥5x_i\ge 5xi​≥5.

For a fixed variable, say x1≥5x_1\ge 5x1​≥5, let x1′=x1−5x_1'=x_1-5x1′​=x1​−5. Then x1′+x2+x3+x4=2.x_1'+x_2+x_3+x_4=2.x1′​+x2​+x3​+x4​=2. Number of solutions: (2+4−13)=(53)=10.\binom{2+4-1}{3}=\binom{5}{3}=10.(32+4−1​)=(35​)=10. There are 4 choices of variable, so subtract 4⋅10=40.4\cdot 10=40.4⋅10=40.

Now check intersections: if two variables are each at least 555, their sum is at least 10>710>710>7, impossible. So no intersections occur.

Thus N(7)=120−40=80.N(7)=120-40=80.N(7)=120−40=80.


5. Count solutions for sum 111111

We need the number of solutions of x1+x2+x3+x4=11,0≤xi≤4.x_1+x_2+x_3+x_4=11, \qquad 0\le x_i\le 4.x1​+x2​+x3​+x4​=11,0≤xi​≤4.

Total nonnegative solutions: (11+4−13)=(143)=364.\binom{11+4-1}{3}=\binom{14}{3}=364.(311+4−1​)=(314​)=364.

Subtract solutions with at least one variable ≥5\ge 5≥5

For a fixed variable, say x1≥5x_1\ge 5x1​≥5, let x1′=x1−5x_1'=x_1-5x1′​=x1​−5. Then x1′+x2+x3+x4=6.x_1'+x_2+x_3+x_4=6.x1′​+x2​+x3​+x4​=6. Number of solutions: (6+4−13)=(93)=84.\binom{6+4-1}{3}=\binom{9}{3}=84.(36+4−1​)=(39​)=84. With 4 choices: 4⋅84=336.4\cdot 84=336.4⋅84=336.

Add back intersections of two variables each ≥5\ge 5≥5

Suppose x1≥5x_1\ge 5x1​≥5 and x2≥5x_2\ge 5x2​≥5. Put x1′=x1−5,x2′=x2−5.x_1'=x_1-5,\quad x_2'=x_2-5.x1′​=x1​−5,x2′​=x2​−5. Then x1′+x2′+x3+x4=1.x_1'+x_2'+x_3+x_4=1.x1′​+x2′​+x3​+x4​=1. Number of solutions: (1+4−13)=(43)=4.\binom{1+4-1}{3}=\binom{4}{3}=4.(31+4−1​)=(34​)=4. Number of pairs of variables: (42)=6.\binom{4}{2}=6.(24​)=6. So add back 6⋅4=24.6\cdot 4=24.6⋅4=24.

Triple intersections

If three variables are each at least 555, the sum is at least 15>1115>1115>11, impossible. So no further terms.

Hence N(11)=364−336+24=52.N(11)=364-336+24=52.N(11)=364−336+24=52.


6. Total count

Therefore the required number of matrices is N(3)+N(5)+N(7)+N(11)=20+52+80+52=204.N(3)+N(5)+N(7)+N(11)=20+52+80+52=204.N(3)+N(5)+N(7)+N(11)=20+52+80+52=204.

7. Comparison with stored answer

Our derived answer is 204\boxed{204}204​ which matches the stored correct answer.

PreviousNext

More from Permutations and Combinations

  • If 2n+1Pn−1​:2n−1Pn​=11:21, then n2+n+15 is equal to :2023 · Numerical
  • In an examination, there are 5 multiple choice questions with 3 choices, out of which exactly one is correct. There are 3 marks for each correct answer, − 2 marks for each wrong answer and 0 mark if the question is not attempted. Then,…2022 · Numerical
  • The number of 7-digit numbers which are multiples of 11 and are formed using all the digits 1, 2, 3, 4, 5, 7 and 9 is ​.2022 · Numerical
  • The letters of the word 'MANKIND' are written in all possible orders and arranged in serial order as in an English dictionary. Then the serial number of the word 'MANKIND' is ​.2022 · Numerical
  • The number of 3-digit odd numbers, whose sum of digits is a multiple of 7, is ​.2022 · Numerical
  • Let A be a 3 × 3 matrix having entries from the set {− 1, 0, 1}. The number of all such matrices A having sum of all the entries equal to 5, is ​.2022 · Numerical
  • The total number of three-digit numbers, with one digit repeated exactly two times, is ​.2022 · Numerical
  • The number of 5-digit natural numbers, such that the product of their digits is 36 , is ​.2022 · Numerical