title: Calculate mod in casio fx-991ES PLUS, answer given is wrong
body: <p>when I'm using my calculator to calculate mod, until now it have been giving me right answers, until I reached this problem that said:
  5^-1 mod 18 = 11
  my calculator kept given the answer to be number one instead.
  Can this kind of problems (where b > a) be solved using a calculator??</p>
  
  <p>*please note that I'm using the calculator in ab/c mode and it's solving problems fine until now.</p>