 
<translate> Ajụjụ. Otu ọkpọkọ nwere ike imejupụta olulu mmiri n'ime nkeji iri abụọ ebe olulu mmiri ahụ nwere ike ịgbafu site na mmiri na-agbafu mmiri n'ime nkeji iri isii. Mgbe e meghere paịpụ abụọ ahụ, chọpụta mgbe olulu mmiri ahụ ga-ejupụta? Nhọrọ: (A) Nkeji iri na asaa (B) Nkeji 87 (C) Nkeji iri atọ (D) Nkeji iri abụọ na asaa (E) Nkeji 11 Zaa ya. (C) ihe Olee ihe e kwuru nke na-egosi na azịza ya bụ eziokwu? <eng> Question. A pipe can fill a cistern in 20 minutes whereas the cistern when fill can be emptied by a leak in 60 minutes. When both pipes are opened, find when the cistern will be full?
Options:
(A) 17 minutes
(B) 87 minutes
(C) 30 minutes
(D) 27 minutes
(E) 11 minutes
Answer. (C)
What step-by-step reasoning justifies that answer? 