I'll start you off (tell me if I've got some kind of misconception),Originally Posted by askmemath
you know two points on the graph: (0,10) and (80,28)
use that information to find the equation.
once you have the equation, substitute 20 for y (or f(x), whichever you prefer) and solve for x
to find the load that makes the spring reach it's limit, substitute 40 for y and solve for x