Find the 20th term from the last term of the AP : 3, 8, 13, . . ., 253
a = 3, d = 5
Now, 253 = a + (n + 1) d
⇒ 253 = 3 + (n -1) x 5
⇒ 253 = 3 + 5n – 5 = – 2
⇒ 5n = 253 + 2 = 255
⇒ n = 255/5 = 51
Therefore, 20th term from the last term = 51 – 19 = 32
a32 = a + 31d
= 3 + 31 x 5
= 3 + 155 = 158
Thus, required term is 158