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Q.

Let b1b2b3b4 be a 4-element permutation with bi{1,2,3,,100} for 1i4 and bibj for ij, such that either b1,b2,b3 are consecutive integers or b2,b3,b4 are consecutive integers. Then the number of such permutations b1b2b3b4 is equal to __________.

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answer is 18915.

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Detailed Solution

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bi{1,2,3100}  Let A= set when b1b2b3 are consecutive  n(A)=97+97++9798 times =97×98  Similarly when b2b3b4 are consecutive  n(B)=97×98n(AB)=97+97+9798 times =97×98  

 Similarly when b2b3b4 are consecutive 

n(B)=97×98n(AB)=97n(AUB)=n(A)+n(B)n(AB)  Number of permutation =18915

 

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Let b1b2b3b4 be a 4-element permutation with bi∈{1,2,3,……,100} for 1≤i≤4 and bi≠bj for i≠j, such that either b1,b2,b3 are consecutive integers or b2,b3,b4 are consecutive integers. Then the number of such permutations b1b2b3b4 is equal to __________.