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Permutations and Combinations question

2022 · Shift 1 · Q25
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Permutations and Combinations question

2022 · Shift 1 · Q25

JEE AdvancedMathematicsPermutations and CombinationsNumerical+3 / −1
The number of 4-digit integers in the closed interval [2022, 4482] formed by using the digits 0,2,3,4,6,70,2,3,4,6,70,2,3,4,6,7 is ‾\underline{\hspace{2cm}}​.
Numerical answer
View written solutionFree

Correct answer: 569

  1. We need to count all 4-digit integers in the interval [2022,4482][2022,4482][2022,4482] using only the digits {0,2,3,4,6,7}.\{0,2,3,4,6,7\}.{0,2,3,4,6,7}.

  2. Since the number must lie between 202220222022 and 448244824482, let the 4-digit number be abcdabcdabcd. We count valid numbers casewise by the thousands digit.


Case 1: Thousands digit = 222

Then the number is of the form 2bcd2bcd2bcd. To be in [2022,4482][2022,4482][2022,4482], it must satisfy 2bcd≥2022.2bcd \ge 2022.2bcd≥2022.

All digits must come from {0,2,3,4,6,7}\{0,2,3,4,6,7\}{0,2,3,4,6,7}.

Total numbers starting with 222:

  • bbb: 666 choices
  • ccc: 666 choices
  • ddd: 666 choices

So total is 63=216.6^3=216.63=216.

But some are less than 202220222022. Since the first digit is already 222, compare with 202220222022:

  • If b=0b=0b=0, then compare the last two digits with 222222.
    • If c=0c=0c=0, then any ddd gives numbers 200d<2022200d < 2022200d<2022. This contributes 666 invalid numbers.
    • If c=2c=2c=2, then we need d≥2d\ge 2d≥2 to be valid. So only d=0d=0d=0 is invalid. This contributes 111 invalid number.
    • If c∈{3,4,6,7}c\in\{3,4,6,7\}c∈{3,4,6,7}, then number is already >2022>2022>2022.

Thus invalid numbers below 202220222022 are 6+1=7.6+1=7.6+1=7.

Hence valid numbers in this case: 216−7=209.216-7=209.216−7=209.


Case 2: Thousands digit = 333

Any number of the form 3bcd3bcd3bcd is automatically between 202220222022 and 448244824482.

Choices for each of b,c,db,c,db,c,d are 666 each, so count is 63=216.6^3=216.63=216.


Case 3: Thousands digit = 444

Now the number is of the form 4bcd4bcd4bcd, and must satisfy 4bcd≤4482.4bcd \le 4482.4bcd≤4482.

We count valid possibilities by comparing with 448244824482.

Subcase 3.1: b<4b<4b<4

Allowed digits less than 444 from the set are 0,2,3.0,2,3.0,2,3. So bbb has 333 choices. Then c,dc,dc,d can be anything from the set: 666 choices each.

Count: 3⋅6⋅6=108.3\cdot 6\cdot 6=108.3⋅6⋅6=108.

Subcase 3.2: b=4b=4b=4

Now compare 44cd≤448244cd \le 448244cd≤4482.

So we need cd≤82.cd \le 82.cd≤82. Both digits must be from {0,2,3,4,6,7}\{0,2,3,4,6,7\}{0,2,3,4,6,7}.

  • If c<8c<8c<8, then since available digits for ccc are all from the set, all possible c∈{0,2,3,4,6,7}c\in\{0,2,3,4,6,7\}c∈{0,2,3,4,6,7} are less than 888. For each such ccc, any ddd is allowed. That gives 6⋅6=36.6\cdot 6=36.6⋅6=36.

But when c=8c=8c=8 would need d≤2d\le 2d≤2, impossible since 888 is not available. So no extra restriction arises.

Thus count here is 363636.

Hence total for thousands digit 444 is 108+36=144.108+36=144.108+36=144.


  1. Therefore total count is 209+216+144=569.209+216+144=569.209+216+144=569.

So the required number of integers is 569.\boxed{569}.569​.


Comparison with stored answer

Stored correct answer = 569569569.

Our derived answer is also 569569569, so they agree.

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