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


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