function ret=interest( P, r, t )
  %% P = Principal
  %% r = interest rate (as a decimal)
  %% t = time in years
  ret = (P*( 1 + r/12 )^(12*t));
  ret*=100;
  ret+=.5;
  ret=floor(ret);
  ret/=100;
  ret-=P;
endfunction
