1.  The sum of how many terms of the series 6 + 12 + 18 + 24 + ... is 1800 ?
A. 16
B. 24
C. 20
D. 18
E. 22

Answer: Option B

Explanation:

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.