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JEE Advanced 2022 · Permutations and Combinations

The number of 4-digit integers in the closed interval [2022, 4482] formed by using the digits 0, 2, 3, 4, 6, 7 is ______.

Answer: 569569

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Written solution

Digits allowed: 0,2,3,4,6,7\text{Digits allowed: } 0, 2, 3, 4, 6, 7

Range: 2022≤N≤4482\text{Range: } 2022 \le N \le 4482

To find: number of 4-digit numbers N\text{To find: number of 4-digit numbers } N

N is 4-digit⇒first digit ≠0N \text{ is 4-digit} \Rightarrow \text{first digit } \neq 0

N≤4482⇒first digit∈{2,3,4}N \le 4482 \Rightarrow \text{first digit} \in \{2, 3, 4\}

Plan: split by the first digit\text{Plan: split by the first digit}

First digit∈{2,3,4}\text{First digit} \in \{2, 3, 4\}

Case d₁ = 3: 3abc 3000 ≤ 3abc ≤ 3777

count=63=216\text{count} = 6^3 = 216

Case d₁ = 2: 2abc ≥ 2022 ⇒ abc ≥ 022 All 2abc = 6³ = 216 Exclude 2abc < 2022 ⇔ abc < 022

2000 to 2007:6 values2000\text{ to }2007: 6 \text{ values}

2020:1 value2020: 1 \text{ value}

total excluded=7\text{total excluded} = 7

d1=2: 216−7=209d_1 = 2:\ 216 - 7 = 209

Case d₁ = 4: 4abc ≤ 4482 ⇒ abc ≤ 482

a∈{0,2,3}: 3×62=108a \in \{0, 2, 3\}:\ 3 \times 6^2 = 108

a=4: 44bc≤4482 always⇒62=36a = 4:\ 44bc \le 4482 \text{ always} \Rightarrow 6^2 = 36

d1=4: 108+36=144d_1 = 4:\ 108 + 36 = 144

209+216+144=569209 + 216 + 144 = 569

3×216=6483 \times 216 = 648

648−7−72=569648 - 7 - 72 = 569

(7<2022, 72>4482)(7 < 2022,\ 72 > 4482)

569\boxed{569}

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