s = 2500
r1 = 8/100/12
r2 = 18/100/12
q = 1500

n = 36

f[d_] := Module[{p = s},
  Do[p = p (1 + If[p > q, r2, r1]) - d, {n}];
  Abs[p]]

ListPlot[Table[{x, f[x]}, {x, 85, 90, 0.01}]]

ListPlot[Table[{x, f[x]}, {x, 87.38, 87.4, 0.001}]]