यदि $${}^1{P_1} + 2.{}^2{P_2} + 3.{}^3{P_3} + .... + 15.{}^{15}{P_{15}} = {}^q{P_r} - s,0 \le s \le 1$$, तो $${}^{q + s}{C_{r - s}}$$ के बराबर है ______________।