Originally Posted by **tttcomrader**

I came across this problem just now:

The mass of the part of a rod that lies between its left end and a point x meters to the right is 5x^{2} kg. The linear density of the rod at 8 meters is ___kg/meter and at 4 meters the density is ___kg/meter.

Now, I understand that meter is x while kg is f(x), since density is kg/meter, the density of of the rod at 8m should be f(8)/8, which is 320/8 = 40 kg/m. But the answer turns out to be incorrect, would anyone please explain how to get the right answer?

Thank you.