A mail carrier randomly inspects every 20th letter being mailed. Out of 600 letters in the sample, 3 were open. There were 18,000 letters being mailed. Make an inference about the number of all the letters being mailed that were open.

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.