In a class of 20 students, 40% are boys. 25% of the boys and 50% of the girls wear glasses. how man students wear glasses?

20 students.

40% are boys. (40/100)*20 = 0.4 * 20 = 8 boys.
Girls= 20 - 8 =  12 girls.

25% of the boys wear glasses. = (25/100)*8 = 0.25*8 = 2 boys wear glasses.
50% of the girls wear glasses. = (50/100)*12 = 0.50*12 = 6 girls wear glasses.
   
Total students that wear glasses = 2 + 6 = 8 students.
8 students wear glasses.