Density of a Spring

Here is the following excersise one teacher posed as a challenge to solve:

A spring has one side attached to the wall and the other one free.The spring has at rest
length L, and linear mass density ρ(x)= constant. Suppose we pull the spring to a new length L'.
1)Find the new mass density of the spring ρ'(x).

Now after trying it at home with a spring it is clear to me that the mass density of the spring becomes smaller as we get from the wall to the other end (after i have streched it). So there must be an explanation to this...