You may have noticed Think Tank answer sheets coming home periodically this school year.   Think Tank, which aids students in developing number sense and computation strategies, has become a favored math center.   Think Tank cards are organized categorically and increase in difficulty as students progress through the tank. Students select from among 20 cards in each level, completing the math problem on the card.  Once all 20 cards have been completed, students advance to the next level.  Working at an individual pace, each student is responsible for tracking his or her progress using a chart kept in his or her math folder.