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

Sn = n/2 * (a1 + an)

Where:
Sn = sum of the series
n = number of terms
a1 = first term
an = last term

In this case, the first term a1 = 1, the last term an = 2x - 1, and the sum Sn = 400. We need to find the value of x.

The number of terms in the series can be calculated using the formula for the nth term of an arithmetic series:

an = a1 + (n-1)d

Where:
an = nth term
a1 = first term
d = common difference

In this case, the common difference is 2, so the nth term an = 2x - 1. Substituting the values, we get:

2x - 1 = 1 + (n-1)2
2x - 1 = 1 + 2n - 2
2x = 2n
x = n

Now, we substitute the values into the sum formula:

400 = n/2 * (1 + 2n - 1)
400 = n/2 * 2n
400 = n^2
n = 20

Therefore, the value of x is 20.