In an A.P. 1st term is 1 and the last term is 20. The sum of all terms is = 399 then n = . . .
Choose the correct alternative answer.
In an A.P. 1st term is 1 and the last term is 20. The sum of all terms is = 399 then n = . . .
(A) 42 (B) 38 (C) 21 (D) 19