GIVEN:Numbers are from 11 to 120. FORMULA USED: Sum of first ‘n’ natural number =
n(n+1)
2
CALCULATION: Sum of all numbers from 11 to 120 = Sum of first 120 natural numbers – Sum of first 10 natural numbers. Sum of all numbers from 11 to 120 =
120(120+1)
2
−
10(10+1)
2
= 7260–55 = 7205 Alternate Method We know, nth number = 1st number + (n - 1)d ⇒ 120=11+(n−1)×1 ⇒ 109=n−1 ⇒ n=110 .Sum of n numbers, Sn=