David is watching maria and alma race electric trains around a track. maria's train can go around the track in 10 seconds. alma's train can go around the track in 12 seconds. david wonders, if the trains start at the same time, how many seconds will it take for both trains to cross the finish line together the first time? david needs to find the of 10 and 12. to find this he could list of both 10 and 12 and circle each number that appears in both lists. he should then identify the number that is circled in both lists