五一班的人数=五年级人数x25%
而五年级人数=全校人数x1/5
所以
=全校人数x1/5x25%
=全校人数x1/5x1/4
=全校人数x1/20
=全校人数x5%
答:五一班人数占全校学生人数的百分之五.