题目

Pollution inside homes and other buildings kills more than 4 million people each year．Many people die as a result of breathing smoke or from cooking over wood﹣powered of coal﹣powered stoves．One way to reduce the number of deaths is through cooking equipment powered by the sun．Crosby Menzies，a solar power expert in a South African company，described his latest solar cooker called the“Sol.4”，”It is four square meters of mirrors，six to eight meters in length．It is quite a large cooler．”The“Sol.4”works by reflecting （反射）light from the sun off large mirrors若正多边形的中心角为20°，那么它的边数是    ．