Question 230820
(1/(1+r))*(30/r)-100 = 0 
(1/(1+r))*(30/r) = 100
Multiply thru by r*(1+r)
30 = 100r*(1+r)
30 = 100r^2 + 100r
10r^2 + 10r - 3 = 0
*[invoke solve_quadratic_equation 10,10,-3]
------------
Sub r for x