Let the first term, common difference and number of terms of an AP are a, d and n respectively.
Given that, first term (a) = 12.
Now by condition,
7th term (T7) = 11th term (T11) - 24
[∵ nth term of an AP, Tn = a + (n - 1)d]
⇒ a + (7 - 1)d = a + (11 - 1)d - 24
⇒ a + 6d = a + 10d - 24
⇒ 24 = 4d ⇒ d = 6
∴ 20th term of AP = 12 + 19 × 6 = 126