\(\frac{4}{5} v_b \times \rho_w \times g=v_b \times \rho_b \times g\)   \(\left\{\begin{array}{l}\text { Where, } \\ v_b=\text { volume of block } \\ \rho_w=\text { density of water }=1000\,kg / m ^3 \\ \rho_b=\text { density of block }\end{array}\right.\)

\(\Rightarrow \frac{\rho_w}{\rho_b}=\frac{5}{4}\)

\(\rho_b=\frac{4}{5} \times 1000=800 \,kg / m ^3\)

And when block is put in liquid of density \(\rho_f\) it just floats

\(\text { So, } v_b \times \rho_b \times g=v_b \times \rho_l \times g\)

\(\Rightarrow \rho_b=\rho_l\)

\(\text { So, } \rho_l=800 \,kg / m ^3\)