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.