五天中任意三天都行，且与顺序无关，所以P=C3，5*0.2*0.2*0.2*0.8*0.8=10*0.2*0.2*0.2*0.8*0.8=0.0064*0.8=0.00512
(5*4*3)/(3*2)*0.2*0.2*0.2*0.8*0.8
10*0.2*0.2*0.2*0.8*0.8
0.0512
