You can use the formula k = F/x to find the spring constant. k is the spring constant, F is force and x is extension. Put the values in (in SI of course) and you get k = 20/0.05 and get 400N/m as the spring constant. It is important to note, however, that some sites which I have seen describe the formula as k = -F/x, and some as k = F/x. In school I remember learning the second one, though I don't know which is correct