Missing number is 6. All oppositenumbers add up to 21.
缺少的数字是6,每组相对数字的和为21。 。
2
Both Numbers may slide synchronously in opposite directions.
两数可以同步地向相反方向移位。
3
When the direction is judged the feedback - sector Numbers must be identified. When the minus feedback is formed, the factors in the opposite direction should be odd number.