What would be the dimensions of a new right rectangular prism that has 20 fewer unit cubes than the original prism

The complete question in the attached figure

Part 1) What is the volume of the prism?
[volume of the prism]=H*L*W
where
L=length of the cube
W=wide of the cube
H=height of the cube

we know that
L= 4 units
W=3 units
H=5 units
[volume of the prism]=5*4*3
[volume of the prism]=60 units³

the answer Part 1) is 60 units³

Part 2) · What would be the dimensions of a new right rectangular prism that has 20 fewer unit cubes than the original prism?

original volume=60 units³
new volume=(60-20)--------> 40  units³
remember that
[original volume of the prism]=(5*4*3)--------> (5*4)*(2+1)
[original volume of the prism]=(5*4*2)+(5*4*1)
[original volume of the prism]=40 units³+20 units ³
40 units³---------> new volume of the prism
therefore
the dimensions of a new right rectangular prism are-------> (5*4*2)

the answer Part 2) is 
the new dimensions are
L= 4 units
W=2 units
H=5 units