Answer:
The pizza which is the better buy in each situation is:
-  The 6-inch diameter pizza for $5.
 
Explanation:
It is given that:The 6-inch diameter pizza for $5.
The 10-inch diameter pizza for $8.99.
This means that the cost for 1-inch diameter will be:
 $ (8.99/10) = $ 0.899
Also, the 6-inch diameter pizza for $5.
This means that the cost for 1-inch diameter will be:
 $ (5/6)= $ 0.833
Since, 0.833 < 0.899
This means that the 6-inch diameter pizza for $5 is cheaper than 10-inch diameter pizza for $8.99.
  Hence, the better buy would be: 
 The 6-inch diameter pizza for $5.