How many terms of A.P -10, -7, -4, -1, ........ must be added to get the sum-104?

Given:
-10, -7, -4, -1, ........
Clearly,

a = -10, d = 3


⇒ -2 × 104 = n[-20 + 3n – 3] ⇒ -208 = n (3n – 23)
⇒ 3n ² – 23n + 208 = 0
⇒ 3n ² – 39n + 16n + 208 = 0
⇒ 3n (n – 13) + 16 (n – 13) = 0
⇒ (n – 13) (3n + 16) = 0

⇒ n = 13,

∵ it cannot be a fraction ∴ n = 13
Hence 13 terms must be added to get the sum of the A.P – 104.