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Permutations and Combinations question

2024 · 9 Apr · Shift 2 · Q58
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Permutations and Combinations question

2024 · 9 Apr · Shift 2 · Q58

JEE MainMathematicsPermutations and CombinationsNumerical+4 / −1
The number of integers, between 100 and 1000 having the sum of their digits equals to 14 , is ‾\underline{\hspace{2cm}}​.
Numerical answer
View written solutionFree

Correct answer: 70

Let the three-digit integer be abc‾\overline{abc}abc, where:

  • aaa is the hundreds digit, so 1≤a≤91 \le a \le 91≤a≤9
  • b,cb,cb,c are the tens and units digits, so 0≤b,c≤90 \le b,c \le 90≤b,c≤9

We need: a+b+c=14a+b+c=14a+b+c=14

We must count the number of digit triples (a,b,c)(a,b,c)(a,b,c) satisfying these conditions.

1. Remove the lower bound on aaa

Since a≥1a\ge 1a≥1, define x=a−1x=a-1x=a−1 Then x≥0x\ge 0x≥0 and x≤8x\le 8x≤8.

The equation becomes: (x+1)+b+c=14(x+1)+b+c=14(x+1)+b+c=14 x+b+c=13x+b+c=13x+b+c=13

Now we count non-negative integer solutions of x+b+c=13x+b+c=13x+b+c=13 with digit restrictions: x≤8,b≤9,c≤9x\le 8,\quad b\le 9,\quad c\le 9x≤8,b≤9,c≤9

2. Count all non-negative solutions without upper bounds

The number of non-negative integer solutions of x+b+c=13x+b+c=13x+b+c=13 is (13+3−13−1)=(152)=105\binom{13+3-1}{3-1}=\binom{15}{2}=105(3−113+3−1​)=(215​)=105

3. Subtract invalid cases

We now subtract solutions where any variable exceeds its allowed maximum.

Case 1: x≥9x\ge 9x≥9

Let x′=x−9≥0x'=x-9\ge 0x′=x−9≥0 Then x′+b+c=4x'+b+c=4x′+b+c=4 Number of solutions: (4+3−12)=(62)=15\binom{4+3-1}{2}=\binom{6}{2}=15(24+3−1​)=(26​)=15

Case 2: b≥10b\ge 10b≥10

Let b′=b−10≥0b'=b-10\ge 0b′=b−10≥0 Then x+b′+c=3x+b'+c=3x+b′+c=3 Number of solutions: (3+3−12)=(52)=10\binom{3+3-1}{2}=\binom{5}{2}=10(23+3−1​)=(25​)=10

Case 3: c≥10c\ge 10c≥10

Similarly, number of solutions: 101010

4. Check overlaps

  • x≥9x\ge 9x≥9 and b≥10b\ge 10b≥10 would require at least 9+10=19>139+10=19 > 139+10=19>13, impossible.
  • x≥9x\ge 9x≥9 and c≥10c\ge 10c≥10 impossible.
  • b≥10b\ge 10b≥10 and c≥10c\ge 10c≥10 impossible.

So there are no overlaps to add back.

5. Final count

Thus required number is 105−15−10−10=70105-15-10-10=70105−15−10−10=70

Therefore, the number of three-digit integers whose digits sum to 141414 is 70\boxed{70}70​

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