question: Mary was working in a chemistry lab with a mixture of chemicals that was 90% water and weighed 20 pounds.  After returning to the lab from a weekend break, she calculated the mixture was now 50% water.  How much does the mixture now weigh? For purposes of this puzzle, assume the non-water part of the mixture was not affected by evaporation. 
choices.label: A B C D E
choices.text: kilograms dollar time period water dog 12 pounds