甲班男生是乙班女生的4/5 => 甲男4k 乙女5k
乙班男生是甲班男生7/8 =>乙男7m 甲男8m => 4k=8m => k=2m
甲=乙=(5k+7m)=17m =>甲女=17m-8m=9m
=>甲乙两班女生人数比=9m :5k=9m:10m=9:10