Formulate the following requirements in goal programming form:

a$)5x+10y$ should be approximately equal to $50$

b$)5x+10y$ should preferably not exceed $50$

c$)5x+10y$ should preferably be at least equal to $50$

d$)5x+10y$ should be as close as possible to $50$ but not higher than that

e$)5x+10y$ should be as close as possible to $50$ but not lower than that