Question 44642
The distance (d) a spring will stretch varies directly as the force (f) applied to the spring. If a force of 12lb is required to stretch a spring 3inchs, what force is required to stretch the spring 5 inches?

If d varies directly with f the math way of saying this is 
d=kf where k is a constant of proportionality.
In these problems you are usually given enough information 
to find k, as follows:

3=k(121)
k=3/121
So for your spring the proper equation relating distance and force
is d=(3/121)f

Now you are asked what force is required to stretch the spring 5 inches.
So, 5=(3/121)f
Multiply both sides by 121 to get:
605=3f
Divide both sides by 3 to get:
201.67= f
 
That is the required force.
Cheers,
Stan H.