Answer:
Width is 12 inches and height is 4 inches.
Step-by-step explanation:
Let the width of the piece of cardboard be = w inches
Then length is =
inches
As given, 4 inch squares are cut from each corner;
So, the width is =
inches
Length is =
inches
When we are folding the cut portion, that gives the height as = 4 inches
The volume is given by : length x width x height
And volume is given as 256 cubic inches
So, 
=> 
=> 
=> 
or 
=> 
=> 
We get ,
; w = -8(neglect the negative value)
; w = 12 inches
And length =
inches
Hence, dimensions are : L = 24 inches ; w = 12 inches and h = 4 inches
We can check by putting these values in equation : 

=> 
=> 
6/-2 = -18/6 = -3
The appropriate choice is ...
A) -3
4 1/5 + B = 9 3/5
21/5 + B = 48/5
B = 27/5
B = 5 2/5
Letter C is the correct answer!
Hope this helps! ;)
The way you typed it, it seems like you need to know the total number of beads, which would be 300