Answer :

To find: 20th term from the last

In the given AP

First term,a = 3

common difference, d = 5

we know, nth term of an AP is

a_{n} = a + (n - 1)d

Now, Let the no of terms in given AP is n

last term = 253

⇒ 253 = a + (n - 1) d

⇒ 253 = 3 + (n -1) x 5

⇒ 253 = 3 + 5n – 5 = – 2

⇒ 5n = 253 + 2 = 255

⇒ n = 255/5 = 51

Total number of terms in AP is 51

Therefore, 20th term from the last term will be 32th term from starting (as, 51 - 20 + 1 = 32)

a_{32} = a + 31d

= 3 + 31 x 5

= 3 + 155 = 158

Thus, required term is 158

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