Explanation:
Let the spring constant of two springs are k₁ and k₂. In series combination of springs, the equivalent of springs is given by :
[tex]k_{eff}=\dfrac{1}{\dfrac{1}{k_1}+\dfrac{1}{k_2}}[/tex]
So, if it is required to swap out a single spring for the two springs that are in series, the spring constant need to be to generate the same force at any displacement is,
[tex]\dfrac{1}{\dfrac{1}{k_1}+\dfrac{1}{k_2}}[/tex]
Hence, this is the required solution.