五年级三个班人数相等.一班男生和二班女生相等,三班男生是全部男生的2/5,全部女生是全年级的几分之几

设x一班男生x人,二班男生y人,全部男生z人,三班女生a人
x+y=3/5z,(2/5)z+a=x+y,(2/5)z+a=3/5z,a=1/5z,x+y+a=3(x+y)-z,
x+y+1/5z=3(x+y)-z,1/5z=2(x+y)-z,z=10(x+y)-5z,6z=10(x+y),z=5/3(x+y)
a=1/3(x+y) [x+y+1/3(x+y)]/3(x+y)=(4/3)(x+y)/3(x+y)=4/9
全部女生是全年级的4/9