Thinking users interactive system

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I'm trying to solve this exercise but I am not sure about the correctness.

Exercise: Consider the following measurement data for an interactive system:

  • measurement interval: 5 min
  • number of users: 50
  • number of servers: 10
  • average response time per transaction: 20 sec
  • Dmax: 1 sec/transaction (the largest service demand at any single center)
  • Dtot: 2 sec/transaction (the sum of the service demands at the centers)
  • Number of completed transactions: 75

On average, how many useres are thinking?

This is what I tried:

Nnot-thinking = R * X = R * C / T = 5
Nthinking = N - Nnot-thinking = 45

Where:

  • R is the response time
  • X is the throughput
  • C is the number of completed transaction
  • T is the measurement interval
  • Nnot-thinking is the number of users that are not thinking
  • N is the number of users
  • Nthinking is the number of thinking users