The sum of how many terms of the series 6 + 12 + 18 + 24 + ... is 1800 ? Choose the most appropriate option.
A) 20  B) 24  C) 28  D) 32  E) 24
This is an A.P. in which a = 6, d = 6 and Sn = 1800   Then, n [2a + (n - 1)d] = 1800   2   n [2 x 6 + (n - 1) x 6] = 1800   2   3n (n + 1) = 1800   n(n + 1) = 600   n2 + n - 600 = 0   n2 + 25n - 24n - 600 = 0   n(n + 25) - 24(n + 25) = 0   (n + 25)(n - 24) = 0   n = 24   Number of terms = 24.   24)
<result>B</result>
The sum of how many terms of the series 6 + 12 + 18 + 24 + ... is 1800 ?