The average age of 30 students is 9 years .If the age of their teacher is included ,it becomes 10 years . The age of the teacher (in years ) is :
A) 27 B) 31 C) 35 D) 40

• 31*10-30*9=40
• 11 years agoHelpfull: Yes(10) No(1)
• average of a new person= previous average+(increase in average *number of persons including new person)
  age= 9 +(1*31)
  age=40
• 10 years agoHelpfull: Yes(1) No(0)
• let x be the sum of students and
  x/30=9
  x=270
  let y be the age of teacher
  (270+y)/31=10
  y=310-270=40
• 10 years agoHelpfull: Yes(1) No(0)
• D) 40
  solution: let total of 30's student age is x so average is 9 of 30 student so 9*30=270
  now with their teacher total of age is now x+teacher= 10*31= 310
  now if we want teacher age then 310 (total of 30 student's age) - 270 = 40
• 9 years agoHelpfull: Yes(1) No(0)
• suppose teacher age x.
  30*9+1*x=31*10
  x=40
• 9 years agoHelpfull: Yes(0) No(0)
• when the age of teacher is included then the average age of each students is increased by 1year
  so total increase is 30*1=30
  
  hence age of teacher's=30+10=40
• 9 years agoHelpfull: Yes(0) No(0)
• option:D
  the total age of 30 stu=30*9=270
  the total age of 30 stu & a teacher=31*10=310
  the age of teacher in years=310-270=40
• 9 years agoHelpfull: Yes(0) No(0)
• Ans is option D
• 8 years agoHelpfull: Yes(0) No(0)
• 30x9=270
  31x10=310
  310-270=40
• 5 years agoHelpfull: Yes(0) No(0)
• 310-270=40
• 5 years agoHelpfull: Yes(0) No(0)