To find the sum of the arithmetic series 2+4+6+...+20, we can use the formula for the sum of an arithmetic series:

S = (n/2)(a + l)

where S is the sum, n is the number of terms, a is the first term, and l is the last term.

In this case, the first term a = 2, the last term l = 20, and the common difference d = 2 (since each term increases by 2).

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

l = a + (n-1)d

Substituting the values, we have:

20 = 2 + (n-1)2
20 = 2 + 2n - 2
20 = 2n
n = 10

Now we can substitute the values into the sum formula:

S = (10/2)(2 + 20)
S = 5(22)
S = 110

Therefore, the sum of the arithmetic series 2+4+6+...+20 is 110.