S(10) = a + (n-1)d
34 = a + 9d ---- Eqn 1
Sum(20) = n/2(2a +(n-1)d)
710 = 20/2(2a + 19d) ----- Eqn 2
Solve these two equations and find the value of a and d and put the values in the first eqn (a + (n-1)d) to find the 25th term
n = the term
a = the starting number
*d = the difference in the series for each number
