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JEE Advanced 2022 Paper 2 · Mathematics · Permutations and Combinations · medium

A shopkeeper distributes 12 identical sweets among 4 distinct children so that each child gets at least 1 and at most 5 sweets. The number of ways to…

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A shopkeeper distributes 1212 identical sweets among 44 distinct children so that each child gets at least 11 and at most 55 sweets. The number of ways to do this is
  1. A. 70
  2. B. 85
  3. C. 125
  4. D. 165
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Answer: (B) 85

Let aia_i be the number received by child ii and put bi=ai−1b_i=a_i-1, so b1+b2+b3+b4=8b_1+b_2+b_3+b_4=8 with 0≤bi≤40\le b_i\le4. Without the upper bound there are (113)=165\binom{11}{3}=165 solutions. A violation means some bi≥5b_i\ge5; setting bi=5+cib_i=5+c_i leaves cc-sum 33, giving (63)=20\binom63=20 solutions per child, and 4⋅20=804\cdot20=80 in total. Two children cannot both violate since 5+5>85+5>8, so no overlap correction is needed. The answer is 165−80=85165-80=85. The option 165165 ignores the upper limit, and 125125 arises if the minimum of 11 is dropped but the maximum of 55 is kept (counting solutions with 0≤ai≤50\le a_i\le5).
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