The sum of the fourth and twelfth term of an arithmetic progression is 20. What is the sum of the first 17 terms of the arithmetic progression?  Choices:
A) 300 B) 120 C) 150 D) 170 E) 270

Rationale: "n th term of a . p . is given by a + ( n - 1 ) d 4 th term = a + 3 d 12 th term = a + 11 d given a + 3 d + a + 11 d = 20 - - > 2 a + 14 d = 20 - - > a + 7 d = 10 sum of n term of a . p = n / 2 [ 2 a + ( n - 1 ) d ] subsitiuing n = 17 . . . we get 17 / 2 [ 2 a + 14 d ] = 17 [ a + 7 d ] = 17 * 10 = 170 . . . answer is d . . ."
Answer: D