42) c¹⁹: c¹¹ c²⁰·c⁵: c¹⁸ =

42) $$ \frac{c^{19}: c^{11}}{c^{20} \cdot c^5: c^{18}} = \frac{c^{19-11}}{c^{20+5} : c^{18}} = \frac{c^8}{c^{25}: c^{18}} = \frac{c^8}{c^{25-18}} = \frac{c^8}{c^7} = c^{8-7} = c^1 = c$$

Ответ: c