Calculating Loads in Two Co-Axial Helical Springs

[Alwightmush]

Homework Statement

2. Two helical springs of the same initial length are made from the same circular section wire and have the same number of coils. They are assembled co-axially and are compressed between two parallel plates by a force of 450 N. The mean coil diameters of the outer and inner springs are 100 mm and 75 mm respectively. Determine the load in each spring.
(Answer: 316 N, 134 N)

Homework Equations

F = (Τ x Pi x d^3) / (8 x D) ?

3. Own working
I tried using the figures in the above equation to calculate values for the Shear Stress T but wasn't sure how to separate the loads into a part for each spring.

Question as above, answers as well, but I can't figure out how to calculate them as I don't have the correct formula I presume.

Any help would be much appreciated, exam tomorrow looms. :(

 

[redargon]

how about using the formula for the spring constant (the shear stresses will be different in each spring, so you're adding a variable) ie.
k = (G*d^4)/(8*D^3*n)
you know that G,d and n are the same for each and you know the ratios of D.
You should then get to something like:
(0.75^3)F1 + F1 = 450N
and F1 + F2 = 450N
solving simulatneously should get you the answers specified.