first, convert 4/5 as a decimal. I just go online to do that or you can divide 4 by 5 which is 0.8:
then multiply 856 to 0.8:
856 • 0.8 = 684.8
then round because you can’t have one eighth of a student -> = 685
then subtract 856 - 685 = 171
honestly answer depends on the teacher as some teachers will make you round while others won’t, but the correct answer would usually be 171 or 171.2