Ajar of jelly beans that weigh 4.25 ounces costs 2.89. what is the cost of one ounce of jelly

You have: 
Cost in dollars --> $2.89 
Total ounces --> 4.25 

But you want to know the per unit cost for *1* ounce. 

You have the right method. You want cost ($) per ounce so put the price over the amount in ounces. 
2.89 / 4.25 = x / 1 

If you notice, since the denominator is 1, this is simply: 
x = 2.89 / 4.25 

So the unit cost is just $2.89 divided by the number of ounces (4.25). 

Plug that into your calculator and you get: 
0.68 

That's in dollars. So if you want it in cents, move the decimal point 2 places to the right. 

 
$0.68 per ounce 
or 
68 cents per ounce