Three quantities P, Q and R are such that PQ = KR, where K is a constant. When P is kept constant, Q varies directly as R; when Q is kept constant, P varies directly as R and when R is kept constant, P varies directly as Q. Initially, P was at 25 and P : Q : R was 1 : 9 : 25. Find the value of P when Q equals 81 at constant R.