Answer:
The questions seems incomplete, so I will answer it in a general way. I suppose you have a table with the average monthly rainfall for each month (from January to December). To compute the sum of them just add together all of the average monthly rainfall.
For the case of the table attached, the sum is:
7.57 + 9.6 + 28.45 + 73.8 + 141. 31 + 267.6 + 314 + 275.1 + 336.47 + 171. 71 + 15.3 + 3.94 = 1644.85 mm
