The sum of the fourth and twelfth term of an arithmetic progression is 20. What is the sum of the first 12 terms of the arithmetic progression? Choose the correct choice from the following 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 = 12 . . . we get 12 / 2 [ 2 a + 14 d ] = 15 [ a + 7 d ] = 12 * 10 = 120 . . . answer is b . . ."
Answer: B