A phone company offers two monthly plans. Plan A costs $16 plus an additional 0.14 for each minute of calls. Plan B costs 21 plus an additional 0.10 each minute of calls. For what amount of calling do the two plans cost the same?

Given: ae ≅ ce ; de ≅ be prove: abcd is a parallelogram. we have that ab || dc. by a similar argument used to prove that △aeb ≅ △ced, we can show that △ ≅ △ceb by. so, ∠cad ≅ ∠ by cpctc. therefore, ad || bc by the converse of the theorem. since both pair of opposite sides are parallel, quadrilateral abcd is a parallelogram.