Circle 0
http://nlvm.usu.edu/en/nav/frames_asid_122_g_3_t_1.html?open=instructions&from=topic_t_1.html

The purpose of this applet is to promote addition, logic, and problem solving skills. The goal of the activity is to find sums of 21 using the integers that are given in a set of circles that are intertwined. Students who successfully can makes sums of 21 in each of the circles complete the activity.