SOLUTION: The first number in the set of 11 consecutive multiples of 3 is 21. What is the middle number in the set?

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Question 1144821: The first number in the set of 11 consecutive multiples of 3 is 21. What is the middle number in the set?
Answer by ikleyn(52793) About Me  (Show Source):
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The middle number is the 6-th term in this sequence.


a%5B6%5D = 21 + (6-1)*3.


Calculate.


I used the standard formula for the n-th term of an arithmetic progression.

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For introductory lessons on arithmetic progressions see
    - Arithmetic progressions
    - The proofs of the formulas for arithmetic progressions
    - Problems on arithmetic progressions
    - Word problems on arithmetic progressions
in this site.

Also,  you have this free of charge online textbook in ALGEBRA-II in this site
    - ALGEBRA-II - YOUR ONLINE TEXTBOOK.

The referred lessons are the part of this online textbook under the topic "Arithmetic progressions".


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Free of charge online textbook in ALGEBRA-II
https://www.algebra.com/algebra/homework/complex/ALGEBRA-II-YOUR-ONLINE-TEXTBOOK.lesson

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