The ratio of the number of Ali's pens to the number of Marks pens was 2:3 at first. After Dad gave Ali another 16 pens, the ratio became 2:1. How many pens did Mark have? If Mark gave away 6 pens, what would the new ratio of Ali's pens )after Dad gave her 16 pens) to the number of Mark's pens?

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.