Answer:2.04 kg
Explanation:
Given
mass of the first block [tex]m_1=0.68\ kg[/tex]
Suppose it brings an extension of x units
When a second mass [tex]m_2[/tex] is attached, then extension quadruples i.e. 4x
For the first case
[tex]m_1g=kx\quad \ldots(i)[/tex]
For the second case
[tex](m_1+m_2)g=k(4x)\quad \ldots(ii)[/tex]
divide (i) and (ii)
[tex]\dfrac{m_1}{m_1+m_2}=\dfrac{1}{4}\\\dfrac{0.68}{0.68+m_2}=\dfrac{1}{4}\\\\4\times 0.68=0.68+m_2\\m_2=3\times 0.68=2.04\ kg[/tex]
