If the sum of the 4th term and the 12th term of an arithmetic progression is 12, what is the sum of the first 15 terms of the progression?  Choose the correct choice from the following answers
A) 90  B) 80  C) 70  D) 60  E) 50

Rationale: "4 th term + 12 th term = 12 i . e . , ( a + 3 d ) + ( a + 11 d ) = 12 now , sum of first 15 terms = ( 15 / 2 ) * [ 2 a + ( 15 - 1 ) d ] = ( 15 / 2 ) * [ 2 a + 14 d ] = ( 15 / 2 ) * 12 - - - - - - - - - - - - - - - from ( 1 ) = 90 answer : a"
Answer: A