Answer:
D) 15
Step-by-step explanation:
This is an arithmatic progression.
The formula for the sum of arithmatic progression is
[tex]s = \frac{n}{2} (2a + (n - 1)d)[/tex]
where d is the common difference between successive terms and a is the first term. By applying this formula,
[tex]120 = \frac{n}{2} (2(1) + (n - 1)(1)) \\ 120 = \frac{n}{2} (1 + n) \\ n(1 + n) = 240 \\ n {}^{2} + n - 240 = 0 \\ (n - 15)(n + 16) = 0 \\ n = 15 \: or \: n = - 16(reject)[/tex]
