If the sum of the 4th term and the 12th term of an arithmetic progression is 24, what is the sum of the first 15 terms of the progression? Choose the correct option.
A) 300 B) 240 C) 220 D) 200 E) 180
4th term + 12th term = 24   i.e., (a+3d)+(a+11d) = 24   Now, Sum of first 15 terms = (15/2) * [2a + (15-1)d]   = (15/2) * [2a + 14d]   = (15/2) * 24 --------------- From (1)   =180   Answer : 180
<result>E</result>
If the sum of the 4th term and the 12th term of an arithmetic progression is 24, what is the sum of the first 15 terms of the progression?