1.
Weight of potatoes........Cost
 12 lb ................................. $17
 18 lb ................................. X
by cross multiplication we get
12 X = 17 × 18
12 X = 306
X = 306/12
X = $ 25.5
        
                    
             
        
        
        
The end result will not be the same as the beginning number because of the change in amounts in-between. Here's an example similar but easier
I have two dollars
I get half of it more 
I now have three dollars
I give half of that away
Now I have a dollar and a half
It all depends on that the course if of the numbers
 
        
             
        
        
        
The whole number would be one and the remainder is 5. hope it helps.