The sum of fourth and twelfth term of an arithmetic progression is 20. What is the sum of the first fifteen terms of that arithmetic progression?
A
300
B
120
C
150
D
130
Correct Answer: 150
ধরি, ধারাটির ১ম পদ a এবং সাধারণ অন্তর d a + 3d + a + 11d = 20 2a + 14d = 20 S15 = 15/2 { 2a + ( 15 - 1) d } = 15/2 × ( 2a + 14d) = 15/2 × 20 = 150