42) $$\frac{c^{19} : c^{11}}{c^{20} : c^5 : c^{18}} = \frac{c^{19-11}}{c^{20-5-18}} = \frac{c^8}{c^{-3}} = c^{8-(-3)} = c^{11}$$.
Ответ: $$c^{11}$$