Calculate the Sum of Numbers from 11 to 20: Formula and Solution

The sum of the numbers from 11 to 20 can be found using the formula for the sum of an arithmetic series:

S = (n/2)(a1 + an)

where S is the sum, n is the number of terms, a1 is the first term, and an is the last term.

In this case, n = 10 (since there are 10 terms from 11 to 20), a1 = 11, and an = 20. Substituting these values into the formula, we get:

S = (10/2)(11 + 20) = 5(31) = 155

Therefore, the sum of the numbers from 11 to 20 is 155.