Calculate the density of a rectangular piece of wood from the following data:
length: 25 cm
width: 10cm
height: 4 cm
mass: 15g

There are several information's that are already given in the question. Based on those information's the answer can be easily deduced.
Length: 25 cm 
Width: 10cm 
Height: 4 cm 
Mass: 15g

Volume of the wood = 25 * 10 * 4
                                 = 1000 cubic cm
Now
Density of the wood = Mass / Volume
                                 = 15/1000
                                 = 0.015 gm/cm^3

I hope that the procedure is clear enough for you to understand and this is the answer that has come to your desired help.

Wish i knew this to !