A series S1 of five positive integers is such that the third term is half the first term and the fifth term is 20 more than the first term. In series S2, the nth term defined as the difference between the (n+1) term and the nth term of series S1, is an arithmetic progression with a common difference of 30.
First term of S1 is Choose the correct choice from the following choices:
A) 80
B) 90
C) 100
D) 120
E) 130
Explanation :   According to the question,   S1:- 2a, b, a, c, 2a + 20   And,   S2:- b - 2a, a - b, c - a, 2a + 20 - c   Also,   =&gt; b - 2a + 30 = a - b i.e. 3a - 2b = 30 .   =&gt; a - b + 30 = c - a   =&gt; c - a + 30 = 2a + 20 - c i.e. 3a - 2c = 10   Solving them, we get a = 50, b = 60, c = 70   So series S1: 100, 60, 50, 70, 120 and series S2: -40, -10, 20.   Answer : 100
<result>C</result>
A series S1 of five positive integers is such that the third term is half the first term and the fifth term is 20 more than the first term. In series S2, the nth term defined as the difference between the (n+1) term and the nth term of series S1, is an arithmetic progression with a common difference of 30.
First term of S1 is