you consider the arithmetic series with first term 30, common difference 1, and last term 80
the sum of arithmetic sequence is n/2(first term + last term), and there are (80-30)+1 = 51 numbers so n = 51 and so 51/2(30+80) = 2805 which represents the sum 30 + 31 + 31 + ... + 78 + 79 + 80
