 
 <translate> Consider the question. Two pipes A and B can fill a cistern in 20 and 30 minutes respectively, and a third pipe C can empty it in 40 minutes. How long will it take to fill the cistern if all the three are opened at the same time?
Options:
(A) 17 1/9
(B) 17 1/3
(C) 17 1/7
(D) 17 1/9
(E) 17 1/1
 What is the step-by-step reasoning process to arrive at the answer: (C)? <ibo> Tụlee ajụjụ ahụ. Ngwá ọrụ abụọ bụ́ A na B nwere ike ime ka mmiri jupụta n'olulu mmiri n'ime nkeji iri abụọ na nkeji iri atọ, nke atọ abụrụ C nwere ike ime ka mmiri gbapụ n'olulu mmiri n'ime nkeji iri anọ. Ogologo oge hà aṅaa ka ọ ga-ewe iji jupụta olulu mmiri ahụ ma ọ bụrụ na e meghee olulu mmiri atọ ahụ n'otu oge? Nhọrọ: (A) 17 1/9 (B) 17 1/3 (C) 17 1/7 (D) 17 1/9 (E) 17 1/1 Gịnị bụ usoro e ji atụgharị uche site n'otu nzọụkwụ gaa n'ọzọ iji ruo azịza ya: (C)? 