Help with text boxes

Question

Hi to all of you.I am totaly newbie in c# Visual studio.
ok here is the my problem
i must for homework do next.
i have 5 text box on form.a,b,c,d,e
in all of them i  only must type NUMERIC format not letters.
now i must do next
mathematic > b/a  = M
d/c= N

and then automaticly must show in  e this result.
e=(1-(N/M))*100


can you understand my problem and can you please help me.
thank you. and sorry about bad english,