Find the 20th term from the last term of the AP : 3, 8, 13, . . ., 253.

The 20th term from the last term of an AP: 3, 8, 13, … 253 = the 20th term from the beginning of the AP: 253. 13, 8, 3.
In the A.P.: 253, …, 13, 8, 3, first term = 253 and common difference = 3 – 8 = -5
Therefore, a_20 = a + 19d
⇒ a_20 = 253 + 19(-5) = 253 – 95 = 158