To find the sum of an arithmetic series, we can use the formula:

Sn = (n/2)(a + L)

where Sn is the sum of the series, n is the number of terms, a is the first term, and L is the last term.

In this case, the first term (a) is 11, the last term (L) is 99, and the common difference (d) is 2 (since each term increases by 2).

To find the number of terms (n), we can use the formula:

L = a + (n-1)d

Substituting the given values:

99 = 11 + (n-1)(2)
99 = 11 + 2n - 2
99 - 11 + 2 = 2n
90 = 2n
n = 45

Now we can substitute the values into the sum formula:

Sn = (n/2)(a + L)
Sn = (45/2)(11 + 99)
Sn = (45/2)(110)
Sn = 45 * 55
Sn = 2475

Therefore, the sum of the series 11+13+15+...+95+97+99 is 2475.