Answer
320 Degree Days
Work Step by Step
1. Degree days in January 1 = 23
Degree days in January 2 = 25
The temperature rose 2 degrees for every day
2. Thus the sequence in 23,25,27 ... 41
Use Formula for adding numbers in a series
$Sum_{n} =\frac{n}{2}(a_{1}+a_{n}) $
In other words, add the last number and the 1st number and then multiply that by the total number of numbers in the sequence, and divide that by 2
$Sum_{10} =\frac{10}{2}(23+41) $ = $5\times64$ = 320
