I am not really sure sorry.........
Answer:
the correct option is C i.e. 
Step-by-step explanation:
We need to solve the expression :

Factors of 128 are: 2x2x2x2x2x2x2
and we know √ = 1/2
and √a.b = √a.√b
Solving:

So, the correct option is C i.e. 
So the board is divided into 5 pieces, and 2/5 of the board is left. The leftovers equal 4 originals. This is equal to 4 times 5.
equation:
2/5(5x)=20
2x=20
x=10
He started with 10 boards.
Hope this helps!
Volume=legnth times widht times height
since this is a rectangle and multiplication is commutative
doesn't matter what order wr multiply
V=5*5*12
V=300 in^3