how to write an experssion for this word problem    a restarant chef finds a good deal on trout and decides to...

how to write an experssion for this word problem 

a restarant chef finds a good deal on trout and decides to make it the special .  He buys enough for 80 servings.  The trout is deliverd in multiple shipements to ensure freshness.  After 2 days, he has services 9 orders of trout .  write an expression that means -9/2 times the time plus 80.  How do I write this expression