The average monthly salary of 8 workers and one supervisor in a factory was $430. When the supervisor, whose salary was $870 per month, retired, a new person was appointed and then the average salary of 9 people was $400 per month. The salary of the new supervisor is:
A. $700
B. $600
C. $430
D. $400
E. $500

• Simple really.
  
  The answer is B
  
  When there are 9 people, you're supposed add all the values together and divide by the number of values present
  
  So if there are 9 factors, A B C D E F G H I, I being the superivisor, we have
  
  (A+B+C+D+E+F+G+H+I)/9 = 430
  
  We know the value of I=870
  So, A+...+H+870 == 430x9==3870
  
  A+...+H = 3870-870
  => 3000
  
  Now the new supervisor reduces the average to 400 per month
  
  similar rules apply, except this time we do not know the superivsor salary, but we know the cumilative sum of the workers salary
  
  3000+I/9 == 400
  I= 3600-3000
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