Average = sum of observation % number of observation
If each term is increased or decreased by same number x,
→ New Average = Old Average ± x
If each term is multiplied or divided by x,
→ New Average = Old Average × or ÷ x


Average of consecutive numbers = middle number
Numbers: 9, 10, 11
Average = middle = 10
The average age of 40 students is 20 years. When the teacher’s age is added, the average increases by 1 year. Find teacher’s age.
Solution: