SOLUTION: Four speakers P, Q, R and S will address the meeting with the condition that speaker Q will speak after speaker S. Find the number of ways in which the order of speakers can be pre

Question 1184223: Four speakers P, Q, R and S will address the meeting with the condition that speaker Q will speak after speaker S. Find the number of ways in which the order of speakers can be prepared?
Answer by ikleyn(52832) (Show Source): You can put this solution on YOUR website!
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In this problem, instead of 4 objects, we consider 4-1 = 3 objects,


looking at the pair SQ as one unit.



For 3 objects, we have 3! = 6 possible permutations;

therefore, the answer to the problem's question is  3! = 6.

Solved and explained.

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Hello, the previous solution was done assuming that speaker Q addresses IMMEDIATELY after speaker S.

If the problem means " after, but not necessary immediately after ", then the answer is = = 12:

In half of 4! = 24 permutations, Q follows S; in other half of permutations, S follows Q.

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