⇒ S2 = 11
∴ a2 = S2 – S1 = 11 – 4 = 7
Taking n = 3, we get
⇒ S3 = 21
∴ a3 = S3 – S2 = 21 – 11 = 10
So, a = 4,
d = a2 – a1 = 7 – 4 = 3
Now, we have to find the 25th term
an = a + (n – 1)d
a25 = 4 + (25 – 1)3
a25 = 4 + 24 × 3
a25 = 4 + 72
a25 = 76
Hence, the 25th term is 76.