Which term of the A.P. 3,15,27, 39, ... will be 120 more than its 21st term?

Explanation

`a = 3, d = 15 – 3 = 12`


Let last term be an `a_n = a + (n – 1) d `


`= 3 + (n – 1) 12 `


`= 12n – 9 (i) `


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