Atheater has 35 rows of chairs. the first row has 20 chairs, the second has 22, the third has 24, and so on. in total, the theater seats people.

            Here we have an arithmetic sequence:
            a 1 = 20,  d = 2,  n = 35
            S n = n/2 * ( a 1 + a n )
            a 35 = a 1 + 34 d = 20 + 34 * 2 = 20 + 68 = 88
            S 35 = 35/2 * ( 20 + 88 ) = 35 / 2 * 108 = 1,890
             In total, the theater seats 1,890 people